Car Maintenance
& MOT Preparation

Most serious injuries in home car maintenance come from a few causes: a car falling off its support, burns, fire, and chemicals in the eyes or on the skin. Learn the rules in this chapter before you pick up a spanner.

1.1 The Principles

• Work on firm, level ground: concrete, not gravel, grass, a slope or soft tarmac on a hot day.
• A jack is for lifting, never for holding. The car's weight goes onto axle stands before any part of you goes underneath.
• Let things cool. Exhausts, turbochargers and engine oil stay hot enough to burn long after a run.
• Know what you are touching. Airbags, high-voltage hybrid and electric systems and fuel lines have their own rules (1.5).
• Use your car's own data for jacking points, torque figures, fluids and intervals (chapter 2).
• Stop when you are out of your depth. Each chapter says which jobs belong in a workshop.

1.2 Lifting the Car: Jacks, Axle Stands and Chocks

• Trolley jack: rated comfortably above the load, with a saddle low enough for your car. A rubber saddle pad protects the sill.
• Axle stands: bought in pairs, rated above the load, used with their proper pin. HSE guidance says pins must be of the correct specification (screwdrivers and bolts are not acceptable), stands must be used on a hard, level surface and inspected every year, and their rated capacity must never be exceeded.
• Wheel chocks against the wheels that stay on the ground. HSE guidance is to chock them securely.
• Drive-on ramps are a stable alternative for jobs that do not need the wheels off. Drive up slowly with a helper guiding, apply the parking brake and chock the wheels.

The handbook shows where the car may be lifted, usually reinforced points on the sill seam marked by notches or arrows. Workshop manuals show stronger points for a trolley jack and stands. Never jack under the floor pan, sump, gearbox or a plastic undertray; HSE advises consulting the manufacturer's handbook for the strong points.

1.3 Heat, Moving Parts and Fire

• Hot parts: exhaust manifolds, turbochargers, catalysts and particulate filters run far hotter than the rest of the engine. Let them cool fully before working near them.
• Cooling system pressure: never open the coolant cap on a hot engine. The Highway Code advises waiting until an overheated engine has cooled naturally before removing it.
• Electric fans can start with the ignition off on some cars. Keep hands, ties and hair clear.
• Running engines: keep sleeves clear of the belt; run engines outdoors only (carbon monoxide).
• Fuel: petrol vapour is heavier than air and ignites easily. No smoking, flames or grinding near fuel lines.

1.4 Fluids, Skin and Safe Disposal

Coolant and screenwash. UK Health Security Agency guidance notes that car antifreeze may contain 60 to 100% ethylene glycol and screenwash up to 30%, that swallowing large amounts can damage the heart, lungs and kidneys and can be fatal, and that children have drunk antifreeze mistaking it for a soft drink. It is also attractive to pets. Keep it in its original container, out of reach, and mop up spills at once. If anyone swallows it, get medical advice immediately (NHS 111, or 999 if they are unwell).

Engine oil. The HSE warns that frequent or prolonged contact with used engine oil may cause dermatitis and other skin disorders including, more rarely, skin cancer. Wear nitrile gloves, wash with soap and water, and keep oily rags out of your pockets.

Brake fluid strips paint and irritates eyes. Wear eye protection when bleeding brakes and wash spills off paintwork with plenty of water.

Disposing of waste

• Used oil: it is illegal to pour engine oil into drains or waterways. Take it in a sealed container to a household waste recycling centre; check your council's website for which sites accept oil and filters, and any quantity limit.
• Coolant and brake fluid: check your council's list; never pour them down a drain.
• Lead-acid batteries: it is illegal to send waste vehicle batteries to landfill or incineration. Take them to a recycling centre or back to a retailer or garage.

1.5 Batteries, Airbags, Hybrids and Electric Cars

The Electronics & Automotive Repair handbook, section 1.5, covers these hazards in detail. In short:

• 12V battery: a spanner across the terminals can weld itself, burn you and burst the battery. Disconnect the negative lead first and reconnect it last. Charge in a ventilated space away from sparks. Disconnection can wipe radio codes and settings on some cars: check the handbook.
• Airbags and pretensioners contain pyrotechnic charges. Follow the electronics handbook and manufacturer's procedure before working near them or yellow SRS wiring.
• Hybrid and electric vehicles have high-voltage systems with orange cabling. Never touch, cut or disconnect orange cables. Tyres, screenwash and wiper blades are no different from any other car, but anything involving the high-voltage system needs a trained technician.

You need a few good tools, the discipline to tighten things correctly, and the right information for your exact vehicle.

2.1 The Basic Toolkit

Buy fewer, better tools: cheap ones round off fasteners.

2.2 Torque: Why It Matters and How to Use a Torque Wrench

A bolt holds parts together by stretching very slightly, like a stiff spring. Too little tension and the joint can work loose, which is how wheels come off and calipers drop. Too much and the bolt stretches permanently, the thread strips or the clamped part distorts: unevenly over-tightened wheel bolts can distort a brake disc. Torque, measured in newton metres (Nm), is the turning force used to reach the correct tension.

1. Clean the threads. Unless the manual says otherwise, fit wheel bolts clean and dry: lubrication changes the tension a given torque produces.
2. Run the bolt in by hand, then snug it with a ratchet. On a wheel, work in a diagonal or star pattern with the tyre on the ground.
3. Set and lock the wrench. Pull smoothly at right angles, holding the centre of the grip, and stop at the first click.
4. Wind a click-type wrench back to its lowest setting after use (unless its maker says otherwise), and never use it to undo bolts. Have it checked if it has been dropped.

2.3 Finding the Right Information for Your Car

• The owner's handbook: jacking points, fluids, bulbs, tyre pressures, service schedule and warning lights. Many makers publish handbooks online.
• The tyre pressure label: usually on the driver's door pillar, fuel flap or glovebox lid.
• The VIN: on a plate (often at the base of the windscreen), stamped into the body and in the V5C log book. It identifies the exact build, which is what parts suppliers need.
• Workshop manuals and technical data: independent manuals and data services give procedures and torque figures for popular models; manufacturers also sell access to official repair information.
• Registration look-ups on parts websites are convenient but can miss mid-year changes and options. Compare old and new parts before fitting.
• GOV.UK: the free MOT history and vehicle recall checks (chapter 11).

2.4 Keeping Service Records

A full history protects the car's value, supports warranty claims and tells the next owner what has been done. Keep the service book or digital record, dated receipts showing part numbers and specifications (for example the oil grade and approval), garage invoices, and a simple log of date, mileage and work (Appendix A has a template). If you service the car yourself, photograph the new parts beside the odometer reading.

The Highway Code advises checking fluid levels at least weekly, and tyre pressures weekly when the tyres are cold.

3.1 Engine Oil: Level and Condition

Park on level ground and follow the handbook's method, which is usually with the engine off and either cold or a few minutes after switching off. Pull out the dipstick, wipe it, push it fully home, withdraw it and read the level between the marks. The handbook says how much oil the gap represents. Top up a little at a time with the correct oil (5.2), rechecking as you go. Do not overfill: excess oil can be churned by the crankshaft and forced past seals. Some cars have no dipstick and show the level on the instrument display instead.

• Dark brown or black: normal for used oil, especially in a diesel.
• Milky or creamy: water or coolant in the oil. A little under the filler cap after short winter trips can be condensation, but milky oil on the dipstick with coolant loss can mean a failed head gasket.
• Smells of fuel, or level rising: fuel dilution, which can follow repeated interrupted particulate filter regenerations (8.3). Change the oil and find the cause.
• Level falling steadily: burning or leaking oil. Look for leaks and blue exhaust smoke (12.6).

3.2 Coolant

The level in the translucent expansion tank should sit between MIN and MAX when cold. Top up with the specified coolant at the correct dilution (5.6). A level that keeps falling means a leak: look for crusty deposits or damp patches at hoses, radiator, water pump and the heater pipes at the bulkhead. A damp front footwell carpet suggests a leaking heater matrix.

3.3 Brake Fluid

Read the level on the master cylinder reservoir without removing the cap. It falls slowly as the pads wear, which is normal. A sudden drop, or a level below MIN, suggests a leak, and the car should not be driven until it is checked: the Highway Code warns that low brake fluid may result in brake failure and a crash. Top up only with the grade printed on the cap (5.7), from a freshly opened container, after cleaning the cap.

3.4 Screenwash and Power Steering Fluid

The MOT checks that the washers deliver enough fluid for the wipers to clear the screen, so an empty bottle or a blocked jet is a failure. Use proper screenwash at the dilution on the label for the season, not washing-up liquid. In winter, a stronger mix stops the fluid freezing; the label gives the protection for each dilution.

Many modern cars have electric power steering with no fluid. Hydraulic systems have a small reservoir with level marks, sometimes with HOT and COLD ranges. Use only the specified fluid. A whine on full lock often means the level is low.

3.5 Lights and Wipers

Walk round the car with the lights on, or use the reflection in a garage door or window. Check sidelights, dipped and main beam, fog lights, brake lights (including the high-level light, with a helper), indicators and hazards on both sides, reversing lights and number plate lights. Lenses should be clean and uncracked. The Highway Code requires lights, indicators, reflectors and number plates to be kept clean and clear. Replace wiper blades that are split, hardened, smearing or juddering; a blade that does not clear the screen is an MOT failure.

3.6 Warning Lights at Start-up

When you switch on the ignition, most warning lights illuminate briefly as a check, then go out once the engine starts. The Highway Code advises that if they do not go out, or come on while driving, you should stop and investigate. A light that never comes on can hide a fault. The MOT checks that the engine malfunction, ABS, airbag and stability control lights, among others, work and are not showing a fault. Appendix B describes the common symbols.

Four hand-sized patches of rubber connect the car to the road. Tyres are one of the few safety-critical parts an owner can check completely, and one of the commonest causes of MOT failures and roadside penalties.

4.1 The Law on Tread Depth and Condition

The Highway Code (Annex 6) states that cars, light vans and light trailers must have a tread depth of at least 1.6mm across the central three-quarters of the breadth of the tread and around the entire circumference. Regulation 27 of the Road Vehicles (Construction and Use) Regulations 1986 also prohibits tyres with certain cuts reaching the ply or cords, lumps, bulges or tears, exposed cords, and tyres unsuitable for the use or not inflated to suit it.

The MOT inspection manual applies the minimum to the primary grooves: those containing the tread wear indicators, or cut as deep when new. Shallower grooves and sipes do not count, and the outer eighth on each side may be bald. A tyre below the minimum is a dangerous defect.

Measuring: use a tread depth gauge in a main groove at several points across the tyre and at three or four places round it. Tread wear indicators are small bars in the base of the main grooves, marked on the shoulder by a triangle, "TWI" or a logo. When the tread is level with them, the tyre is at the limit.

Damage: look for sidewall bulges (the structure has failed, often after a kerb or pothole strike; replace the tyre), cuts to the cords, cracking from age, and damaged valves. Do not pull out nails or screws: a tyre specialist will say whether a repair is permissible, which depends on the size and position of the damage.

4.2 Tyre Pressures

Tyres must be inflated to the vehicle manufacturer's specification for the load being carried. The figures are on the door pillar, fuel flap or glovebox label and in the handbook, often with a higher setting for full loads. The pressure moulded on the tyre is its maximum, not your car's figure. Check when cold, as warm tyres read high, and include the spare: space-saver spares often need a much higher pressure.

Under-inflation wears both shoulders, raises fuel consumption and spoils handling; over-inflation wears the centre and reduces grip. Reset a tyre pressure monitoring system (TPMS) as the handbook describes after adjusting pressures. The MOT checks TPMS on passenger cars first used from 1 January 2012, and a malfunctioning system is a major defect.

4.3 Reading the Sidewall

In 205/55 R16 91V: 205 is the width in millimetres; 55 the sidewall height as a percentage of width; R means radial; 16 is the rim diameter in inches; 91 is the load index (615kg per tyre); V is the speed symbol (240km/h, about 149mph). Replacements should match the size, with a load index and speed rating at least those specified for the car. Insufficient load capacity for the axle is a dangerous MOT defect, and different sizes or structures on the same axle are major defects.

The last four digits of the DOT code give the week and year of manufacture: 3523 is week 35 of 2023. The MOT manual notes that a three-digit code means the tyre is over ten years old. The code may be on one side only, so check the inner sidewall too.

4.4 Tyre Age

Rubber hardens and cracks with age even when tread is deep. There is no legal age limit for car tyres (the ten-year rule in regulation 27 covers heavy goods vehicles, buses and minibuses), but ten years is a sensible yardstick. Michelin advises professional inspection at least yearly once tyres have been in use for five years and replacement ten years after manufacture as a precaution; Continental recommends replacing tyres over ten years old. Check new tyres' date codes before they are fitted.

4.5 Reading Wear Patterns

The Highway Code notes that excessive or uneven wear may be caused by faults in the braking or suspension systems, or wheels out of alignment.

4.6 Wheels and Changing a Wheel

Check alloy wheels for cracks and buckles after any hard impact; cracked wheels must be replaced. Use the bolts designed for each type of wheel (steel and alloy often differ), keep the locking wheel nut key with the jack and note its reference code. Rotate tyres only as the handbook allows and respect any rotation arrow.

A routine service is mostly fluids, filters and inspection, and much of it is within reach of a careful owner. This chapter covers the common jobs and says where home servicing should stop.

5.1 Service Intervals and Schedules

Every manufacturer publishes a service schedule with intervals in miles and in time ("every 12,000 miles or 12 months, whichever comes first"). The time limit matters for low-mileage cars, because oil, coolant and brake fluid deteriorate with age. Variable or "long-life" schedules let the car's computer call for a service and usually need a specific long-life oil. Short trips, towing and stop-start driving often count as "severe" use, which shortens intervals: read your schedule's definition. "Interim" and "full" services vary between garages, so ask for the checklist. Reset the service indicator afterwards using the handbook's sequence or a scan tool.

5.2 Engine Oil Specifications Explained

An oil container carries three kinds of information, and the handbook tells you what your engine needs for each.

• Viscosity grade (SAE), such as 5W-30. The "W" (winter) number describes cold flow: the lower it is, the more easily the oil circulates just after a cold start, when most wear happens. The second number describes viscosity at operating temperature. Modern engines are designed for specific, often very thin, grades such as 0W-20; a thicker oil "for protection" is not an improvement.
• ACEA category. The European manufacturers' association publishes oil sequences. A/B oils, for petrol and diesel engines, have conventional levels of sulphated ash, phosphorus and sulphur (SAPS). C oils have lower SAPS and are designed to be compatible with catalytic converters and diesel particulate filters. Categories carry a number and year, such as C3; the current edition is 2023, which added C7 for very low-viscosity 0W-16 oils.
• API category. The American Petroleum Institute classes petrol engine oils as "S" plus a letter; API SQ, licensed from March 2025, is current. API matters most for American and many Asian engines.
• Manufacturer approval, such as VW 504 00 / 507 00 or MB-Approval 229.51, shows the oil has passed that maker's own tests. "Suitable for" or "meets the requirements of" is not the same as approved. For a car in warranty or on a long-life schedule, use an oil that states the approval.

5.3 Changing the Engine Oil and Filter

5.4 Air and Cabin Filters

The engine air filter is usually a panel in a clipped plastic box. Note which way round it sits, wipe dust out of the box without letting any fall into the intake, and make sure the lid seals all round: unfiltered air causes wear and can damage the airflow sensor. The cabin (pollen) filter, often behind the glovebox or under the scuttle panel, cleans air entering the cabin. A blocked one slows demisting and causes musty smells. Fit it with the airflow arrow as the manual shows.

5.5 Spark Plugs and Glow Plugs

Petrol engines need new spark plugs at the scheduled interval, which is long for platinum and iridium plugs. Work on a cold engine, because aluminium heads are easily damaged. Remove the coils or leads by their bodies, not the wires, and clean dirt from the plug wells before undoing the plugs. Many modern plugs are pre-gapped: only set the gap if the manufacturer says so. Start each new plug by hand so it cannot cross-thread, then tighten to the specified torque or angle.

Diesel glow plugs do not need routine replacement and can seize in the head. A snapped glow plug is an expensive workshop repair, so leave them to a garage.

5.6 Coolant

Coolant lowers the freezing point and raises the boiling point, but just as importantly its inhibitors protect the aluminium, iron, copper and plastics in the system. The inhibitors are used up over time, hence the replacement interval. The main inhibitor technologies are IAT (inorganic, silicate-based, found in older cars), OAT (organic acid, longer-life) and HOAT (hybrid, organic acids with some silicate or phosphate).

Dilute concentrate as the handbook directs (often half and half), using distilled or deionised water, or buy it pre-mixed. A hydrometer or refractometer measures the protection level. Some systems are hard to bleed of air and need a specific procedure or a vacuum filler; an airlock can cause overheating, so if the procedure looks involved, use a garage.

5.7 Brake Fluid

Most brake fluid is glycol-based and hygroscopic: it absorbs moisture through hoses, seals and the reservoir vent, and water lowers its boiling point. If fluid boils in a hot caliper, the vapour compresses and the pedal goes soft (6.5). Minimum boiling points are set by the US standard FMVSS 116, which defines the DOT grades; the "wet" figure is measured after controlled humidification.

Use the grade on the reservoir cap or in the handbook, commonly DOT 4 or a low-viscosity DOT 4 variant. Never put silicone DOT 5 into a glycol system; it is only for systems built for it. Because fluid deteriorates with time, manufacturers specify a time-based change; two years is common in UK schedules, but check yours. Renewing it means bleeding the system (6.4).

5.8 Wiper Blades and Bulbs

Wiper blades are sold by length and fitting type. Some cars need the wipers put into a "service position" before the arms lift clear of the bonnet. Lay a towel on the screen while the blade is off: a bare arm snapping back can crack the glass.

Bulbs are identified by codes such as H7, P21W or W5W in the handbook. Do not touch the glass of halogen bulbs. Access varies from a twist-off cap to removing the bumper. Check the beam pattern against a wall afterwards.

5.9 Auxiliary Belt Inspection

The auxiliary belt drives the alternator, often the water pump, and the air conditioning compressor. With the engine off, inspect it for cracks across the ribs, missing chunks, glazing, fraying and oil contamination, and check the tensioner moves smoothly and the pulleys spin quietly. A squeal on start-up or when the air conditioning cuts in usually means a worn belt or weak tensioner. Photograph the routing before removing a belt.

5.10 Timing Belts and Chains

The timing belt or chain keeps the camshafts in step with the crankshaft. On "interference" engines the valves and pistons share space at different moments, so a broken belt lets them collide and can wreck the engine. Belts therefore have a strict interval in miles and years. Chains last longer but can stretch, often announced by a rattle on cold start.

Treat belt replacement as a professional job unless you have the locking tools, the manufacturer's procedure and real experience. The tensioner, idlers and often the water pump are replaced at the same time. If a used car has no proof the belt has been changed, budget for it. Some engines have a "wet" belt running in oil, which can degrade if the wrong oil is used or the interval missed: use exactly the specified oil.

Pad and disc replacement on a conventional car is a reasonable home job for a careful owner with the right tools and data. If you have any doubt about your work, have it checked before you drive.

6.1 How Disc and Drum Brakes Work

The pedal pushes a piston in the master cylinder, helped by a servo. Brake fluid is almost incompressible, so the pressure travels through pipes and flexible hoses to pistons at each wheel, which force friction material against a disc or drum, turning movement into heat. The hydraulics are split into two circuits so that one leak leaves some braking.

Disc brakes shed heat well and are used at the front of almost all cars, where most braking happens because weight shifts forward, and at the rear of many. Drum brakes push shoes outwards inside a drum and survive at the rear of some small cars. Photograph their springs and adjusters before dismantling, and work on one side at a time.

6.2 Inspecting Pads and Discs

• Pads: check both pads on each wheel; the inner one often wears faster and is hard to see. The MOT inspection manual fails a pad worn to its wear indicator or below 1.5mm. Replace well before that.
• Wear warnings: some pads have a metal tab that squeals against the disc; others have an electrical sensor that lights a warning. Sensors are usually single-use.
• Discs: look for a pronounced lip, deep scoring, cracks or heavy rust on the friction face. Light surface rust after rain is normal. The manufacturer's minimum thickness is often stamped on the disc ("MIN TH"); measure across the friction faces with a micrometer or vernier, not on the lip. The MOT fails a disc only if it is significantly and obviously worn, but the manufacturer's minimum is the safe limit.
• Hoses, pipes and leaks: check hoses for cracks and bulging with the pedal pressed, and rigid pipes for corrosion. Corroded pipes are a common MOT failure. Wetness at a caliper or on the inside of a wheel means a leak: do not drive.

6.3 Replacing Pads and Discs

6.4 Bleeding Brakes

Bleeding removes air, which makes the pedal spongy, and is how fluid is renewed. Follow the manual's wheel order: the traditional furthest-first order does not suit every ABS car. In the two-person method a clear tube runs from the bleed nipple into a jar; a helper presses and holds the pedal while you open the nipple briefly and close it, repeating until clean, bubble-free fluid emerges. Keep the reservoir topped up throughout. One-person pressure or one-way-valve kits make it easier. If air has entered the ABS unit, the manufacturer's procedure may need a diagnostic tool: a garage job.

6.5 Brake Fluid Boiling and Brake Fade

On long descents or when towing, constant braking heats everything. Pad fade leaves the pedal firm but the car slows less, often with an acrid smell. Fluid boiling forms vapour in the calipers, and the pedal goes soft or to the floor. Old fluid with absorbed water boils far sooner (compare the wet and dry figures in 5.7), which is why the time-based change matters. Use a lower gear on long descents, brake firmly in bursts rather than dragging the brakes, and if they fade, stop when safe and let them cool fully.

6.6 Parking Brakes

If a lever parking brake needs more clicks than it used to, or the car rolls on a slope, the cables may be stretched or the rear mechanism seized. Adjust only as the manual describes: over-adjustment makes the rear brakes drag. An electronic parking brake fault lights its own warning, is an MOT failure (major defect) while shown, and needs a scan tool that can read the brake system's codes.

Suspension keeps the tyres pressed onto the road and steering points them. Both wear so gradually that you rarely notice the decline. A methodical check finds worn parts long before the MOT tester does.

7.1 How the Parts Fit Together

• Springs carry the weight; shock absorbers (dampers) stop the car bouncing. In a MacPherson strut, the common front layout, spring and damper form one unit that also locates the wheel.
• Control arms locate the wheel, pivoting on rubber bushes at the body end and a ball joint at the wheel end.
• Anti-roll bars resist body lean and connect to the suspension through short drop links.
• Steering: a rack and pinion moves track rods, with inner joints inside rubber gaiters and outer track rod ends. Assistance is electric or hydraulic.
• Wheel bearings are sealed units, replaced rather than adjusted.

7.2 Checking for Wear

The MOT inspection manual expects testers to judge condition from knowledge and experience rather than fixed tolerances, and you can do much the same. A helper is invaluable.

• Steering play (car on its wheels): with the engine running, have a helper rock the steering wheel gently either side of straight ahead while you watch and feel each joint. Movement before the road wheel moves, or a knock you can feel, means wear.
• Visual: split joint boots and rack gaiters, perished bushes, leaking dampers, and broken coil springs (look closely at the lowest coil, where it sits in its seat).
• Wheel raised on stands: rock the tyre at top and bottom for wheel bearing or ball joint play, and side to side for track rod end play; watch which part moves. Spin the wheel and listen for rumbling.
• Bushes and drop links: lever gently against a strong point (never against brake pipes). A clunk or rubber separated from its sleeve means replacement. A knock over small bumps is often a drop link.

Drop links, track rod ends and some complete arms are bolt-on jobs for a competent owner. Pressed-in joints and bushes need a press, and anything affecting the steering geometry needs an alignment check afterwards. Replace drop links, dampers and springs in pairs across an axle.

7.3 Shock Absorbers and Springs

The Highway Code's test: if the car keeps bouncing after you push down on the front or rear, the shock absorbers are worn, and worn shock absorbers can seriously affect handling. Other signs are a floaty ride, nose-diving under braking and cupped tyre wear. A light film of fluid on a damper is not an MOT failure; severe leakage or negligible damping is.

7.4 Wheel Alignment

• Toe: whether the wheels point slightly in or out viewed from above. The setting most often wrong, and the fastest to wear tyres.
• Camber: the wheel's inward or outward tilt viewed from the front.
• Caster: the backward tilt of the steering axis, which gives self-centring.

Usually only toe is easily adjustable. Have alignment checked after steering or suspension work, a heavy kerb strike, uneven tyre wear or an off-centre steering wheel. Replace worn parts first: aligning a car with worn bushes wastes money.

7.5 Power Steering

Hydraulic systems need the correct fluid level (3.4); a whine that grows on full lock suggests low fluid or a worn pump. Electric (EPS) systems have no fluid. A fault lights the steering warning and makes the steering heavy, and an EPS warning showing a malfunction is a major MOT defect. Diagnosis needs a scan tool; low battery voltage can also cause EPS faults (see the electronics handbook on the charging system).

Most repairs to these systems are workshop jobs, but understanding them helps you spot problems early and avoid expensive mistakes.

8.1 The Cooling System

A water pump circulates coolant through the engine and then the radiator, where air carries the heat away. A thermostat stays shut until the engine is warm. Electric fans pull air through the radiator at low speed, an expansion tank takes up coolant as it expands, and the pressure cap raises the boiling point. The heater matrix is a small radiator behind the dashboard.

Squeeze the main hoses when cold: they should be firm but springy, not soft, swollen, cracked or rock-hard.

8.2 The Exhaust System

Gas passes from the manifold through a catalytic converter, on diesels usually a particulate filter, then pipes and silencers hung on rubber mounts. With the engine cold, look for holes, heavy corrosion, broken hangers and sooty marks at blowing joints. The MOT checks the system is secure and complete and not unreasonably loud, and an exhaust leak that could affect emissions is a major defect. Exhaust fumes in the cabin bring a carbon monoxide risk: have any leak near the front, or any smell inside, checked at once. Bolt-on rear sections are often DIY; seized manifold studs, catalysts and welding are garage work.

8.3 Catalysts, Diesel Particulate Filters and the Law

A catalytic converter converts carbon monoxide, unburnt hydrocarbons and (on petrol engines) nitrogen oxides into less harmful gases. A misfire can overheat and ruin it: a flashing engine light on many cars warns of exactly this, so reduce speed and get it checked.

A diesel particulate filter (DPF) traps soot. GOV.UK guidance explains that it is emptied by regeneration, burning the soot off at very high temperature, which happens on longer runs or when the engine computer triggers it. Cars used mainly for short trips may not complete regenerations, and the filter blocks. Signs include a DPF warning light, limp mode, the fan running after switch-off and a raised idle. The handbook explains what the DPF light wants you to do, often a sustained drive.

Legitimate options for a blocked DPF are a forced regeneration by a garage, professional cleaning or replacement. Find the cause too: sensors, EGR faults, injectors, the wrong oil (5.2) or the car's use pattern. The MOT also checks the engine malfunction light and measures petrol exhaust gases or diesel smoke opacity.

8.4 AdBlue

Many Euro 6 diesels use selective catalytic reduction (SCR) to cut nitrogen oxides, injecting a fluid sold as AdBlue (a trademark of the German automotive association VDA), AUS 32 or DEF: 32.5% high-purity urea in demineralised water, made to ISO 22241.

• Its filler, usually with a blue cap, is beside the diesel filler or in the boot. Act on the low-level warning: most cars will not restart once the tank is empty.
• Buy fluid marked ISO 22241. Contamination can ruin the SCR system.
• Never put AdBlue in the fuel tank, or diesel in the AdBlue tank. If it happens, do not start the engine; call for recovery.
• AdBlue freezes at about −11°C; the system has heaters. Wipe up spills, which leave white crystals.

8.5 Clutch, Gearbox and Driveshafts

The MOT does not cover the engine, clutch or gearbox, but early signs of trouble are worth knowing.

• Clutch slip: revs rise without matching acceleration, especially uphill in high gears. Replacement usually means removing the gearbox: garage.
• Judder pulling away, or a rattle at idle that stops when the clutch is pressed: often a worn dual-mass flywheel (common on diesels), clutch or engine mountings.
• Crunching into gear: clutch not releasing fully, or worn synchromesh. Hydraulic clutches often share the brake fluid reservoir.
• Gearbox oil: use only the specified oil. Automatics, dual-clutch boxes and CVTs each need their own fluid, often with scheduled changes and a temperature-controlled fill. Harsh or delayed shifts need prompt professional attention.
• Driveshafts: clicking when turning sharply under power is the classic sign of a worn outer CV joint. Check the rubber CV boots: a split boot is cheap to replace early, and the MOT fails one that no longer keeps dirt out.

Most modern cars have no separate chassis: the body shell carries the suspension, seat belts and steering, so rust in the wrong place is a structural and MOT problem.

9.1 Why Cars Rust

Steel rusts when bare metal meets water and oxygen, and road salt speeds it up. Paint, zinc coatings and wax protect only while intact, so rust starts at stone chips, scratches, seams, door and tailgate bottoms, sills, wheel arches and anywhere mud packs into a lip. Blocked drain holes in doors, sills and the scuttle let water collect and rust panels from the inside.

9.2 Stone Chips and Scratches

Clean and degrease the area. Remove any rust in the chip with a fibreglass pen and treat staining with rust converter. Find the paint code (on a label in a door shut, under the bonnet or in the boot) and buy matching touch-up paint. Prime bare metal, build up thin layers of colour slightly proud of the surface, then lacquer. A touch-up is about protection first; it is rarely invisible.

9.3 Treating Surface Rust

Bubbling paint means rust underneath. Mask the area, then sand or wire-brush back to bright metal a little beyond the visible rust, which spreads under the paint further than it shows. Treat with a rust converter or inhibitor as directed, prime, use a thin skim of filler only for minor imperfections, then colour and lacquer. Wear a dust mask and eye protection, and use solvent-based products outdoors or with good ventilation.

If the metal is holed or crumbles when pressed, it must be cut out and new metal welded in. Filler, fibreglass or tape over a hole hides the problem and is not an acceptable structural repair for the MOT.

9.4 Underbody Care

• Rinse underneath after the gritters have been out, gently, so high pressure does not strip underseal.
• Clear drain holes in doors, sills and the scuttle with a plastic tie.
• Inspect sills, jacking points, floor pans, subframe and suspension mountings and arch edges with a torch and a gentle prod with a blunt plastic tool.
• Protect sound, clean metal with cavity wax in sills and box sections and underbody wax elsewhere. Wax over loose rust seals the corrosion in. Do not bury brake pipes under heavy underseal.

9.5 Corrosion and the MOT

Appendix A of the MOT inspection manual defines prescribed areas: load-bearing parts to which testable items (brakes, steering, suspension, seat belts and other listed equipment) are mounted, plus any load-bearing or supporting structure or panelling within 30cm of the mounting. Corrosion in a prescribed area fails if it has made a hole, if the metal does not feel firm under finger and thumb pressure, or if finger, thumb or the tester's corrosion assessment tool (used to scrape or tap lightly) creates a hole. Elsewhere, corrosion fails only if it affects braking or steering through misalignment or significantly weakens the overall structure.

Structural repairs must appear as strong as the original, with welding extending to sound metal. Patches must be continuously seam welded unless a correctly removed spot-welded panel is replaced by spot, stitch or plug welding. Gas brazing, soldering, adhesive bonding, fibre reinforcement and body filler are not acceptable on load-bearing members, except where the manufacturer's own repair method uses bonding, MIG brazing or riveting.

The MOT is an annual check that a vehicle meets road safety and environmental standards on the day it is tested. It is not a service and not a guarantee of condition for the year ahead. This chapter covers how it works in England, Scotland and Wales; Northern Ireland has its own system run by the Driver and Vehicle Agency, with its own inspection manual.

10.1 When an MOT Is Due

• A car must have its first MOT by the third anniversary of its registration, then every year by the anniversary of its last MOT. Some vehicles, such as taxis and ambulances, must be tested from one year old.
• An MOT lasts a year. You can have the test up to a month (minus a day) before the current certificate runs out and keep the same renewal date. GOV.UK's example: if the MOT runs out on 15 May, the earliest test date that keeps the renewal date is 16 April.
• You cannot drive or park the vehicle on the road once its MOT has run out, except to drive it to a pre-arranged MOT test or to or from somewhere to be repaired. You can be fined up to £1,000 for driving without a valid MOT.
• You cannot renew vehicle tax without a valid MOT (where one is required).
• Vehicles built or first registered more than 40 years ago do not need an MOT, provided no "substantial changes" have been made, but they must still be kept roadworthy.
• The maximum fee a test station can charge for a car is set by the government: at the time of writing, £54.85, with no VAT. Many garages charge less.

Only centres displaying the blue sign with three white triangles can carry out MOTs. GOV.UK offers free MOT reminders.

10.2 Defect Categories and Results

The inspection manual sorts defects into three categories, and testers can also record advisories. The advice in the last column is DVSA's, from the 2018 introduction of the categories:

You can watch the test from a viewing area, but you are not allowed to interrupt the tester. A pass certificate is not a guarantee: the test does not cover the condition of the engine, clutch or gearbox.

10.3 After a Failure: Retests and Appeals

If the car is left at the test centre for repair and retested within 10 working days, the partial retest is free. If you take it away and bring it back to the same centre by the end of the next working day for certain items (including lamps, wipers and washers, mirrors, plates, seats and belts, wheels and tyres, the horn, battery and doors), the retest is also free. Other partial retests within 10 working days can be charged; after that, it is a full test.

If you think a failure is wrong, discuss it with the test centre before any repairs, then complain to DVSA within 14 working days. If a car passed but should have failed, the limits are 3 months for corrosion and 28 days for other defects.

10.4 MOT History and the Inspection Manual

The free GOV.UK MOT history service shows, for any registration number, past MOT results with dates, recorded mileages, failures, minors and advisories. When buying, look for repeated advisories never fixed, mileage that goes backwards and gaps in testing.

DVSA publishes the MOT inspection manual for private passenger and light commercial vehicles on GOV.UK. Written for testers but publicly available, it lists every check and defect category. Its sections for a car are:

Sections 9 and 10 cover buses and coaches and seat belt installation checks, and do not apply to an ordinary car test. Appendix A of the manual deals with structural integrity and corrosion (see 9.5).

10.5 Preparing for the MOT

Many MOT failures are for things an owner could have found and fixed in minutes: bulbs, wiper blades, tyres, an empty washer bottle. A week or two before the test, work through the checklist in Appendix A, which follows the manual's sections. The thresholds most worth knowing are:

• Tyres: 1.6mm across the central three-quarters all round; no bulges or cuts to the cords; matching size and structure across each axle (4.1 to 4.3).
• Brake pads: a fail at the wear indicator or below 1.5mm (6.2).
• Windscreen: in "zone A", a strip 290mm wide centred on the steering wheel within the swept area, damage larger than 10mm fails; elsewhere in the swept area the limit is 40mm. A chip repair can save a screen.
• Warning lights: engine management, ABS, airbag, ESC, EPB, EPS and TPMS lights must come on and go out. A light showing that the passenger airbag is switched off is not a defect.
• Number plates: correct characters and spacing, no tints or films, and no fixing screws that alter how a character looks.
• Fluid leaks: a leak that forms a pool more than 75mm across within 5 minutes fails.
• Smoke: dense blue or clearly visible black smoke fails; a diesel with a DPF should show no visible smoke at all.

On the day, drive for 20 minutes or more beforehand so the engine is fully warm, clear the boot and cabin so the tester can reach the seat belts and spare wheel well, and leave the locking wheel nut key in the car.

GOV.UK sets out the obligations plainly: the vehicle must be registered with DVLA, taxed, have a current MOT if it needs one and be roadworthy, and you must have at least third party insurance covering your use of it. Rules and fines change, so check GOV.UK for the current position.

11.1 Vehicle Tax (VED)

• You must tax a vehicle used or kept on a public road, even if it is exempt and you pay nothing. Use the reference on the V5C, the green new keeper slip or a DVLA reminder.
• You cannot tax a vehicle that needs an MOT unless it has a valid one.
• Tax is not transferred on sale. The buyer must tax the vehicle (or make a SORN) before driving it; the seller is refunded any full months once DVLA is told.
• Electric cars have paid vehicle tax since 1 April 2025.
• From 1 April 2026, vehicles built before 1 January 1986 can be moved into the historic tax class, exempt from payment unless used commercially. You must apply.
• Not taxing on time brings an £80 fine. DVLA scans the register for untaxed vehicles without a SORN, which can be clamped and removed.

11.2 Insurance

• Driving on a road or in a public place without at least third party insurance is illegal. The police can issue a £300 fixed penalty and 6 points, a court can impose an unlimited fine and disqualification, and the vehicle can be seized.
• Under continuous insurance enforcement, a vehicle must be insured even when not driven unless it has a SORN. An uninsured keeper can be fined £100, have the vehicle clamped, impounded or destroyed, and face a court fine of up to £1,000.
• askMID shows whether a vehicle is on the Motor Insurance Database. Declare modifications to your insurer.

11.3 SORN (Statutory Off Road Notification)

Make a SORN to stop taxing and insuring a vehicle kept off the road (in a garage, on a drive or on private land). You must make one if the vehicle is untaxed or uninsured, even briefly; a SORN does not transfer from a previous keeper. It never needs renewing and ends when you tax, sell, scrap or export the vehicle. You can drive a SORN vehicle on a public road only to or from a pre-booked MOT or other test; any other use risks prosecution and a fine of up to £2,500. Failing to make a SORN when required brings an automatic £80 fine.

11.4 Telling DVLA

You must tell DVLA when you:

• change your address or name (address changes are usually free online; not reporting one risks a fine of up to £1,000);
• sell, transfer, scrap or export the vehicle. Do it online straight away: DVLA treats failure to notify a change of keeper as a registration offence, and until it is told you remain the registered keeper;
• change the vehicle: colour, engine size, fuel type (including an electric conversion), seating capacity, or replacement of the chassis or bodyshell;
• have or develop a medical condition that affects driving.

If a police officer asks, you must be able to show your driving licence, insurance certificate and MOT certificate (if required), or take them to a police station within 7 days.

11.5 Safety Recalls

Manufacturers recall vehicles and parts with serious safety problems, and you will not usually pay for the repair. They write to the registered keeper, another reason to keep your V5C current. The free GOV.UK vehicle recall check shows, by registration number, safety recalls not yet checked or fixed; parts and accessories, including tyres and child seats, can be checked too. Report a serious safety defect (a design or manufacturing fault likely to cause injury, happening suddenly and without warning) to the manufacturer, and to DVSA if you are unhappy with the response.

11.6 Roadworthiness Between MOTs

An MOT certificate does not make a car legal for a year. You are legally responsible for keeping it safe to drive whenever it is driven, and can be fined up to £2,500, banned and given 3 points for driving a vehicle in a dangerous condition. Defective brakes, steering or tyres are a specific offence carrying 3 points (4.1).

Most faults announce themselves through a noise, a smell, a warning light or a change in how the car feels. Notice when the symptom happens (cold or hot, braking or accelerating, turning left or right, at what speed) and you will save time for yourself or a garage in finding the cause. Electrical faults, including flat batteries, charging and starting problems, are covered in the Electronics & Automotive Repair handbook, section 7.3.

12.1 Noises

12.2 Smells

The Highway Code advises investigating any unusual smell, such as burning rubber, petrol or an electrical fault, immediately: do not risk a fire.

12.3 Warning Lights and Fault Codes

On most cars red means stop safely and investigate, amber means attention needed soon, and green or blue lights are information. Appendix B describes the common symbols; the handbook is the authority.

• Oil pressure (red oil can): stop the engine as soon as it is safe and check the level. If the level is right and the light stays on, do not restart: running without oil pressure can ruin an engine quickly.
• Coolant temperature or brake warning (red): stop; see 12.5 or check the fluid and parking brake. Do not drive on with a persistent brake warning.
• Charging (red battery): the alternator is not charging; see the electronics handbook, 5A.
• Engine management (amber engine): steady, get the codes read soon; flashing, on many cars a catalyst-damaging misfire, so reduce load and get it checked promptly.

EU Directive 98/69/EC phased in European on-board diagnostics (EOBD), so petrol cars from about 2001 and diesels from about 2004 have a standard 16-pin socket, usually under the dashboard. An inexpensive scan tool reads generic engine codes such as P0301 (cylinder 1 misfire). A code says what the computer noticed, not which part failed: a misfire can be a plug, coil, injector or compression fault. Note codes before clearing them, and never clear one just to get through an MOT. ABS, airbag and body systems need a more capable tool; the electronics handbook, 5E, covers sensor testing.

12.4 Pulling and Vibration

12.5 Overheating

1. Once cool, check the coolant level. If it is low, top up and look for the leak (8.1).
2. With the engine restarted and warming up, check the cooling fan cuts in as the gauge rises. If not, check the fan fuse and relay (see the electronics handbook) or have it diagnosed.
3. Check the radiator is not blocked by leaves or dirt.
4. If coolant disappears without a visible leak, the oil is milky, or there is white exhaust smoke and bubbles in the expansion tank, suspect the head gasket. Garage.

12.6 Exhaust Smoke by Colour

Preparing for winter, laying a car up properly, buying the right parts and finding a trustworthy garage all save money and trouble.

13.1 Preparing for Winter

The Highway Code's pre-winter advice is to make sure the battery is well maintained and that there are appropriate antifreeze agents in the radiator and windscreen bottle. In autumn:

• Battery: cold cuts output when engines are hardest to turn. If starting is slow, or the battery is several years old, have it tested (see the electronics handbook, 5A, and 5D for batteries that go flat overnight).
• Coolant: check its strength with a hydrometer or refractometer.
• Screenwash: switch to the winter dilution on the label and run the washers until it reaches the jets.
• Wipers and lights: replace smeary blades, free frozen blades before switching on, and check every bulb.
• Tyres: tread depth matters more in slush. Tyres with the three-peak mountain snowflake symbol grip better in cold conditions.

Boot kit: scraper and de-icer (the Highway Code requires you to clear all snow and ice from all windows, keep lights clean and plates legible, and remove snow that could fall into others' path); warm clothes, blanket, torch, phone charger, water and snacks; jump leads or a jump pack; a small shovel; and a warning triangle, which the Highway Code says to place at least 45 metres behind a broken-down vehicle on the same side of the road, but never on a motorway.

13.2 Storing a Car

Laid-up cars suffer flat batteries, flat-spotted tyres, seized brakes and damp. Make a SORN if you will stop taxing and insuring it (11.3). Before storage, wash and wax it, change the oil if it is near due, inflate the tyres to the upper end of the recommended range, and connect a smart battery maintainer (modern cars draw a small current constantly). If it is safe with chocks, leave the parking brake off so the rear brakes do not seize; check the handbook for an EPB. Store under cover with ventilation and a breathable cover, not plastic sheeting. Before driving again, check fluids, pressures and brakes.

13.3 Buying Parts

Registration look-ups are convenient but not infallible: have your VIN ready and compare old and new parts. Avoid unbranded safety-critical parts from marketplace sellers, and keep packaging until the part is fitted and tested.

13.4 Choosing and Using a Garage

• Ask locally, and consider a marque specialist for unusual or complex cars. Garages accredited to The Motor Ombudsman's Service and Repair Code commit to standards on quotes and complaints and offer an independent dispute service.
• Competition rules (the UK Motor Vehicle Block Exemption Order and the Competition and Markets Authority's guidance on it) protect your freedom to have a car in warranty serviced by an independent garage, provided the work follows the manufacturer's schedule with parts of matching quality. Read your warranty terms and keep the invoices.
• Get a written quote, ask to be called before extra work, and ask for the old parts if you want to see them.
• Under the Consumer Rights Act 2015, a trader must perform a service with reasonable care and skill. Raise problems with the garage first, then Citizens Advice or its trade body.

Work through the checklist a week or two before the MOT so there is time to order parts. To print only this appendix, set a page range in your browser's print dialogue.

A.1 Pre-MOT Checklist

Arranged by the sections of the DVSA MOT inspection manual. "Owner" means most owners can check and fix it; "Garage" means repair is normally professional work.

A.2 Service Record Template

Record every job, not only full services, and keep the receipts with it.

Vehicle: registration ____________ · make and model ____________ · VIN ____________ · engine code ____________
Specifications: engine oil grade and approval ____________ · capacity with filter ______ litres · coolant type ____________ · brake fluid grade ______ · tyre size ____________ · tyre pressures front ______ rear ______

Time-based items to track

B.1 Tyre Sidewall Decoder

Common load indices (kg per tyre)

Note that the MOT inspection manual's load index table gives figures for an axle with a tyre at each end (so 1,230kg for index 91), because testers compare it with the axle weight on the manufacturer's plate.

Common speed symbols

B.2 Warning Light Reference

Most dashboard symbols follow the international standard ISO 2575, but shapes and colours vary between makers, and many cars add their own. The descriptions below are of the common generic symbols. Your handbook is the definitive reference. Colours: red, stop safely and investigate; amber, attention needed soon; green or blue, information.