Fault library
Car Fault & Symptom Library
Look up what your car is doing — a noise, a warning light, a smell, a leak or a starting problem — and read what it commonly means, what we check and how urgent it is.
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What is your car doing?
Every entry is written from workshop experience. Symptoms can have several causes, so these pages explain the likely ones rather than promising a single answer.
Starting & electrical
Car won't start and just clicksClicking means the electrical side is trying to work but the engine is not turning over properly. The pattern of the clicking is the most useful clue.Read more Car hard to start when coldCold starting depends on battery condition, fuel pressure, compression and, on diesels, glow plug operation. Cold weather exposes weakness in any of them.Read more Flat battery: why it keeps going flatBatteries do wear out, but a battery that flattens repeatedly usually has help — either it is not being charged properly or something is draining it while the car is parked.Read more Signs your car battery needs replacingMost batteries give warning before they leave you stranded. Recognising it early usually means choosing when the car is off the road rather than being told.Read more Car won't start but the battery is fineWhen the battery tests healthy, the fault is usually in the starting circuit, the fuel side or the sensors the engine needs to run.Read more Car battery keeps going flatRepeatedly flat batteries are rarely bad luck. Something is either draining the battery, failing to charge it, or the battery itself is finished.Read more Starter spins but the engine does not turnA fast whirring or grinding when you turn the key normally means the starter is running but not engaging with the flywheel.Read more
Warning lights
Battery warning light on while drivingThe red battery symbol is a charging-system warning. The engine is normally running on stored battery power only, which is limited.Read more Engine management light flashingA steady amber light and a flashing one are very different. Flashing normally indicates a misfire severe enough to risk expensive damage.Read more DPF warning light onThe DPF light means soot has built up beyond the level the system is happy with. Whether a drive will clear it depends on why it built up.Read more Glow plug (coil) light flashing on a dieselOn many diesels the coil symbol doubles as an engine fault warning. A flashing light usually means a stored fault, not necessarily a glow plug.Read more ABS warning light onWith the ABS light on, the anti-lock function has typically disabled itself. The brakes usually still work, but without anti-lock support in an emergency stop.Read more Brake warning light onThe brake warning light covers several different systems, so the first job is establishing which one has triggered it.Read more Airbag warning light onThe airbag light means the restraint system has logged a fault and may not deploy correctly, so it is worth reading properly rather than clearing.Read more Oil pressure warning light onOf all the dashboard lights, oil pressure is the one to take most seriously — running an engine without pressure can cause damage quickly.Read more
Engine & running faults
Car stuck in limp modeLimp mode is a protection strategy, not a fault in itself. The control unit has seen a reading it does not trust and has reduced power to keep things safe.Read more White smoke from the exhaustColour, smell and whether the smoke clears once warm are what separate normal condensation from a coolant or fuelling problem.Read more Blue smoke from the exhaustBlue smoke points at oil reaching the combustion chamber or exhaust. When it appears — start-up, overrun or under boost — narrows the cause.Read more Black smoke from a diesel exhaustBlack smoke is unburnt fuel. On a modern diesel with a DPF it usually also means extra soot heading into the filter.Read more Car losing power when acceleratingLoss of power is either a fuelling, air or protection issue. Live data under load is what separates them rather than guesswork.Read more Car cutting out while drivingIntermittent cut-outs are among the most important faults to record properly, because the evidence disappears the moment the engine restarts.Read more Car overheating or temperature gauge risingOverheating is one of the few faults where what you do in the next few minutes decides whether it stays a small repair.Read more Engine misfire: symptoms and common causesA misfire means a cylinder is not contributing properly. It is either fuel, spark or compression on that cylinder — and identifying which comes before parts.Read more Car shaking or vibrating at idleIdle is where small faults show up most, because the engine has no load to smooth them out.Read more Turbo failure symptomsTurbos rarely fail without a reason. Oil supply, oil condition and blocked exhaust flow are common contributors, and replacing the turbo alone can repeat the failure.Read more EGR valve symptomsEGR valves soot up over time, particularly on cars used for short journeys. The symptoms overlap with several other faults, so testing matters.Read more Head gasket failure symptomsHead gasket failure is often suspected and less often confirmed. Testing first matters, because several cheaper faults produce similar symptoms.Read more Temperature gauge reading highA gauge climbing above its normal position is an early warning. Acting on it usually costs far less than ignoring it.Read more Car jerking when acceleratingJerking under load usually means the engine is briefly not getting the fuel, air or spark it needs — live data while driving is the quickest route to the cause.Read more
Braking
Grinding noise when brakingGrinding is a metal-on-metal noise and is different from a squeak. It usually means friction material has run out or something is trapped against the disc.Read more Brake pedal feels spongy or goes to the floorA firm pedal is what tells you the hydraulic system is sealed and healthy. Sponginess means something in that system has changed.Read more Steering wheel shakes when brakingWhere and when you feel the shake matters. Vibration only under braking generally points at the braking surfaces; vibration all the time points elsewhere.Read more Squeaky brakesA squeak is not always a fault, but it can be the first sign that pads are near the end of their life. The pattern tells us a lot.Read more
Steering & suspension
Clunking or knocking noise over bumpsSuspension noises are best identified with the car off the ground and the joints loaded, because several components make similar sounds.Read more Car pulling to one sideThe key detail is whether the pull happens all the time, only under braking, or only under acceleration.Read more Heavy steering or a power steering leakSome cars use hydraulic power steering, others electric. The symptoms overlap but the causes and checks are very different.Read more Uneven tyre wearTyres record what the suspension and steering are doing. The wear pattern is often the clearest evidence of an underlying fault.Read more Vibration at motorway speedSpeed-related vibration is nearly always rotational, so we look first at wheels, tyres and driveline rather than the engine.Read more
Noises
Humming or droning noise that changes with speedNoises that follow road speed rather than engine speed usually come from wheels, bearings or tyres. Whether it changes on a bend is the key test.Read more Clicking or knocking noise when turningA click that appears on lock and speeds up as the wheels turn faster is typically driveshaft related rather than a suspension knock.Read more Rattling noise on start-up that goes awayA short rattle from cold that clears once oil pressure builds is a common early warning. Ignoring it can turn a chain job into an engine job.Read more Whining noise when acceleratingThe useful question is whether the whine follows engine revs or road speed, because that separates belt and turbo noises from gearbox and differential noises.Read more Wheel bearing noise: what it sounds likeBearing noise follows road speed, not engine speed, and typically changes as weight moves across the car on a bend.Read more Rattling noise from under the carRattles are located by loading and tapping components with the car on a lift, because a loose shield can sound exactly like something far more serious.Read more Exhaust blowing noiseA blowing exhaust changes the sound of the car and can affect emissions readings and sensor behaviour as well as noise.Read more
Clutch & gearbox
Clutch slipping or revs rising without accelerationA slipping clutch is no longer transferring all of the engine's torque. It normally shows up first in high gear, uphill or under load.Read more Crunching or grinding when changing gearCrunching often means the clutch is not fully releasing rather than the gearbox being worn out — which is worth establishing before any gearbox work.Read more Car judders when pulling awayJudder can be transmission or engine related. Whether it also happens while driving at steady speed helps separate the two.Read more Car will not go into gearWhether it will not select from rest or fights you between gears tells us whether to look at the clutch release system or inside the gearbox.Read more Clutch pedal soft, spongy or stuck to the floorA change in pedal feel is usually hydraulic. It often appears before the clutch stops working altogether, so it is worth acting on.Read more Engine revs but the car does not moveIf the revs rise and the car stays put or barely pulls, drive is being lost somewhere between the engine and the wheels.Read more
Leaks & smells
Coolant leak or a puddle under the carColour and position tell you a lot. Coolant is usually pink, orange, blue or green and smells sweet; oil is dark and greasy.Read more Oil leak under the carOil spreads and drips away from its source, so leaks are traced by cleaning, running and inspecting rather than by looking once.Read more Burning smell while drivingThe type of smell is the diagnosis short cut. Rubber, hot oil, hot brakes and electrical burning are all distinguishable.Read more Smell of petrol in or around the carFuel vapour is a fire risk, so a petrol smell is one of the few symptoms worth acting on the same day.Read more
Heating & air conditioning
Air conditioning not blowing coldAir conditioning loses a little refrigerant over the years, but a system that has gone warm quickly usually has a leak worth finding.Read more Heater blowing cold airThe heater runs on engine coolant, so a cold heater is often the first sign of a cooling system problem rather than a heater fault.Read more Air conditioning smells mustyThe smell normally comes from moisture sitting where it should not, rather than from the refrigerant side of the system.Read more
Longer reads
Repair guides and costs
Our advice articles go further into individual jobs, intervals and what affects the price.
Can't see your symptom?
Describe what the car is doing and send us your registration — we will tell you what it usually means and what checking it involves.