Common Failures
Failure guides for buyers and workshops who want to understand the part before ordering it.
Use these guides to understand the failure pattern, then send the OE number, VIN, photo or vehicle details to check supply.
Common Failures
Range Rover Air Suspension
Why L322, L405 and L494 air springs fail, and what to replace.Read guide
Common FailuresBMW Electric Water Pump
Overheating symptoms, pump connector checks and warranty choices for BMW engines.Read guide
Common FailuresNOx Sensor
AdBlue warnings, limp mode and Position 1 versus Position 2 checks.Read guide
Common FailuresIgnition Coils
Misfires, flashing lights and why German ignition coils are often replaced as sets.Read guide
Common FailuresBMW N20 Timing Chain
Cold-start rattle, revised guides and full-kit checks for BMW N20 chains.Read guide
Common FailuresAlternator Toyota/Nissan
Battery warnings, charging checks and voltage matching for Toyota and Nissan alternators.Read guide
Common FailuresLand Cruiser AHC
Ride-height faults, AHC pump checks and accumulator matching for Land Cruiser.Read guide
Sensors & ElectronicsThe ABS Sensor That Turns One Fault Into Five Warning Lights
A car comes in with five lights on the dash. ABS, traction, stability, the lot. The customer is convinced the brakes are about to fail and he's already half...Read guide
Cooling & ACThe BMW Thermostat That Often Fails Beside the Water Pump
Here's a customer coming back in two months, angry, and it was completely avoidable. He paid you to fix his water pump. Now the car's throwing a...Read guide
Alternators & StartersWhat the Clicking Actually Means When the Car Will Not Start
Turn the key, get a click. Or a machine-gun clicking. Or a horrible grinding. Most people hear all three as the same thing — "car won't start" — but each...Read guide
Cooling & ACOverheats in Traffic but Stays Cool on the Highway: What to Check
If a customer tells you his car runs hot crawling through town but sits dead cool out on the open road, he's just handed you the diagnosis without knowing...Read guide
Sensors & ElectronicsA Lazy Oxygen Sensor Can Cost More Than a Dead One
A dead oxygen sensor is easy money. Code pops up, you swap it, everyone goes home happy. The one that'll drive you mad is the lazy one — still alive, still...Read guide
Air SuspensionWhen One Corner Leaks Down but the Air Spring Is Sound
Car sits down on one corner overnight. Everyone jumps straight to the air spring, swaps it, job done. Two weeks later it's flat on the same corner again —...Read guide
Air SuspensionThe Car Is Not Broken: the Height Sensor May Be Reporting the Wrong Position
One corner sits too high. Or too low. The springs are new, the compressor's fine, there's no leak anywhere — and the thing still won't sit level. Before...Read guide
Diesel EmissionsThe DPF Sensor That Can Be Mistaken for an Expensive Filter Failure
Someone's about to quote a customer a new diesel particulate filter. The filter might be in perfect health. The part that's actually lying costs a tiny...Read guide
Engine ElectronicsThe BMW VANOS Solenoid That Dirty Oil Can Damage
Rough idle, no pull down low, and a fault code muttering something about camshaft timing. The engine sounds sick and the owner's already imagining a huge...Read guide
Engine ElectronicsThe Vacuum Leak Everyone Blames on the Ignition Coils
Rough idle, lean codes, a faint whistle from under the bonnet, and oil quietly vanishing. So somebody sells a set of coils. Then somebody sells spark plugs....Read guide
Cooling & SealingOne Gasket, Two Completely Different Complaints
One customer says his car's leaking oil everywhere. Another says his car's mysteriously drinking coolant. Same car, same engine, same part. This one gasket...Read guide
Diesel ElectronicsHard Cold Start on a Diesel? Check the Module as Well as the Plugs
A diesel that fights you on a cold morning, puffs white smoke, and shakes for the first minute. Everyone grabs glow plugs. Half the time the plugs are...Read guide
Sensors & ElectronicsCam Sensor vs Crank Sensor: Two Different Fault Patterns
Two sensors, near-enough the same name, near-enough the same look — and they fail in completely different ways. Mix them up and you either send the wrong...Read guide
Engine ElectronicsWhy Cleaning a Throttle Body Can Make the Idle Worse
A tech cleans a throttle body. Right call, carbon everywhere, good job. He fires the engine up and now the idle's a mess — hunting, racing, stalling at the...Read guide
Fuel & TurboThe Small Sensor Behind an Expensive Turbo Quote
The scan tool says "boost pressure not reached." Sounds like a turbo, doesn't it. It's often not a turbo at all. It's often the little sensor that measures...Read guide
Alternators & StartersNew Alternator, Belt Still Squeals? Check the Pulley
A workshop fits a brand new alternator. Charging's perfect. And the belt still squeals every cold morning, and the customer's dead sure he was sold a dud....Read guide
Diesel EmissionsWhy an AdBlue or SCR Countdown Can Prevent a Restart
A driver ignores a warning light for two weeks. Now the truck won't start at all, and he's fuming, convinced something's broken. Nothing's broken. The truck...Read guide
Diesel EmissionsThe Temperature Sensor That Can Quietly Damage a DPF
Everyone talks about the DPF pressure sensor. Almost nobody talks about the other one — the temperature sensor that decides whether the DPF is even allowed...Read guide
Sensors & ElectronicsThe Coolant Temperature Sensor That Can Quietly Waste Fuel
A customer says his fuel's gone up, the engine's rough when cold, and the heater's weak. Nobody can find a thing wrong. And nothing is wrong — the engine's...Read guide
Fuel & TurboThe Weak In-Tank Pump That Can Damage the High-Pressure Pump
A customer replaces a high pressure fuel pump. Eight months later it fails again, and now he's certain your parts are rubbish. They weren't. Nobody looked...Read guide
TransmissionThe ZF 8HP Mechatronic: a Computer Living in Gearbox Fluid
A customer gets told he needs a whole new gearbox. Eight grand, maybe more. But the gears are fine. The clutches might well be fine too. The thing that...Read guide
TransmissionxDrive and 4MATIC: Check the Actuator Before Condemning the Transfer Case
A four-wheel-drive warning light. Binding on tight turns. A shudder at low speed. Somebody quotes a transfer case and the number's terrifying. One possible...Read guide
Steering & ChassisWhen Wheel-Bearing Noise Changes with Steering Load
A hum or a growl that gets louder the faster you go. Everyone argues about which side it's on, and half the time they guess wrong and replace the good one....Read guide
Braking & EngineRock-Hard Brake Pedal on a Diesel? Check the Vacuum Supply
A customer says the brake pedal's gone rock hard and he has to stand on it to stop the car. A hard brake pedal can mean the brake-assist system is not...Read guide
Cooling & ACAC Blower Only Works on Full Speed? Check the Resistor or Control Module
The AC works, but only on speed four. One, two and three do nothing at all. In a hot country that's not a minor annoyance — the cabin is either a hurricane...Read guide
Cooling & ACThe AC Receiver Drier Is a Sponge: Why It Is Not Reusable
Every time somebody skips the drier to save fifty bucks, they're gambling with the compressor they just paid for. This is the least glamorous part in the...Read guide
Cooling & ACThe Small Pressure Cap That Controls the Cooling System Boiling Point
The customer says the car keeps losing coolant but there's never a puddle. A head-gasket fault is only one possibility; the cap, tank, hoses, radiator and...Read guide
Cooling & TurboWhy an Auxiliary Coolant Pump May Run After Switch-Off
A customer hears a humming for five minutes after he parks and reckons something's wrong. Nothing's wrong — that noise is what's saving his turbo. And the...Read guide
Electrical & StartingDo You Have to Register a New Battery on a BMW?
Yes. On most BMWs built from around 2002 onwards, a new battery has to be registered to the car with a diagnostic tool. If you skip that step, the car keeps...Read guide
Steering & ChassisWhy Did My Power Steering Suddenly Go Heavy?
Steering that goes heavy suddenly usually means the power assistance has stopped, not that the steering itself has broken. On an older car with hydraulic...Read guide
Brakes & StabilityWhy Is the ESP Light On After a Wheel Alignment?
An ESP or traction control light that appears immediately after a wheel alignment is nearly always the steering angle sensor needing calibration, not a...Read guide
Transmission & DrivelineCan You Replace a Dual Mass Flywheel With a Solid One?
Physically yes, and conversion kits are widely sold. But a solid flywheel does not do the job the dual mass flywheel was doing. The dual mass flywheel...Read guide
Fuel, Turbo & EmissionsWhy Did My New Turbo Fail So Quickly?
A replacement turbocharger that fails within weeks has almost never failed on its own. In the great majority of cases the oil supply was never fixed. A...Read guide
Brakes & StabilityHow Do You Change Rear Brake Pads With an Electronic Handbrake?
On a car with an electronic parking brake you cannot simply wind the rear caliper piston back with a hand tool the way you would on an older car. A small...Read guide
Transmission & DrivelineWhy Does My Automatic Gearbox Shudder at Steady Speed?
A shudder that appears at a steady cruising speed under light throttle, and disappears the moment you press the accelerator harder, is almost always the...Read guide
Engine DiagnosticsWhat Does the Colour of Exhaust Smoke Tell You?
The colour of exhaust smoke tells you which fluid is being burned. Black smoke means unburnt fuel, usually too much fuel or not enough air. Blue smoke means...Read guide
Engine DiagnosticsWhy Is My Engine Burning Oil?
An engine that uses oil without leaking is burning it, and there are four common routes it takes into the combustion chamber: worn or stuck piston rings,...Read guide
Brakes & StabilityWhy Do My Brakes Squeal After New Pads?
New brake pads that squeal are usually the result of a fitting detail being skipped rather than a fault with the pads. Brake squeal is high-frequency...Read guide
Electrical & StartingWhy Does My Car Battery Keep Going Flat?
A battery that keeps going flat has one of three causes: the battery itself is worn out and no longer holds charge, the charging system is not refilling it...Read guide
Cooling & ACWhy Is My Car Air Conditioning Blowing Warm Air?
Car air conditioning that blows warm air most commonly has low refrigerant from a leak, which is the single most frequent cause. Other causes are a...Read guide
Steering & ChassisWhy Does My Steering Wheel Shake When I Brake?
A steering wheel that shakes or shudders when you brake is almost always the front brake discs having uneven thickness, not discs that are literally warped....Read guide
Transmission & DrivelineWhy Is My DSG Gearbox Jerky at Low Speed?
A DSG or dual-clutch gearbox that jerks, hesitates or shudders at low speed is usually the clutches or the mechatronic unit wearing, or on a wet-clutch DSG,...Read guide
Cooling & ACWhy Does My Car Overheat in Traffic But Not on the Motorway?
A car that overheats in slow traffic but runs at normal temperature on the open road has a cooling airflow problem, and the most common cause is the...Read guide
Engine DiagnosticsHow Do I Know If My Car Has a Timing Belt or a Timing Chain?
A timing belt is a toothed rubber belt on the outside of the engine, behind a plastic cover, and it must be replaced at a set interval or it eventually...Read guide
Fuel, Turbo & EmissionsCan You Clean or Block Off an EGR Valve?
A clogged EGR valve can often be cleaned, and that is the sensible first step because it is cheap and frequently restores normal running. If the valve or...Read guide
Engine DiagnosticsHow Often Should I Really Change My Engine Oil?
The long service intervals many modern cars advertise, sometimes twenty or thirty thousand kilometres, are set to lower running costs and assume gentle use,...Read guide
Electrical & StartingWhy Is My Engine Shaking or Misfiring at Idle?
An engine that shakes or misfires at idle usually has one cylinder not firing properly, and the most common causes are worn spark plugs or a failing...Read guide
Steering & ChassisWhat Is That Knocking Noise From My Suspension Over Bumps?
A knocking or clunking noise from the suspension over bumps is most often worn anti-roll bar drop links when it is a light knock or rattle over small bumps,...Read guide
Transmission & DrivelineHow Do I Know If My Clutch Is Slipping?
A slipping clutch is when the engine revs rise but the car does not accelerate to match, because the clutch is no longer gripping firmly enough to transfer...Read guide
Fuel, Turbo & EmissionsWhy Did My Diesel Go Into Limp Mode?
Limp mode, also called limp home or reduced power mode, is the engine computer deliberately limiting the engine to protect it after seeing a reading it does...Read guide
Engine DiagnosticsWhat Should I Do If the Oil Light Comes On?
There are two different oil warnings and they mean very different things. A red oil pressure light, shaped like an oil can, means oil is not reaching the...Read guide
Engine DiagnosticsHow Do I Know If My Head Gasket Has Failed?
A failed head gasket usually shows several symptoms at once, not just one: thick white smoke that lingers and smells sweet, coolant disappearing with no...Read guide
Brakes & StabilityIs It Safe to Drive With the ABS Light On?
With the ABS light on, your normal brakes still work exactly as before, because the hydraulic braking system is untouched. What you have lost is the...Read guide
Steering & ChassisWhy Does My Power Steering Whine When I Turn?
A whining noise from the power steering that rises with engine speed and is loudest at full lock is usually low fluid, which is the first thing to check...Read guide
Fuel, Turbo & EmissionsWhy Is My Car Suddenly Using More Fuel?
A sudden increase in fuel consumption usually has a simple cause, and the free checks should always come first: tyre pressures, a blocked air filter, a roof...Read guide
Electrical & StartingWhy Won't My Car Start If the Battery Is Fine?
What the car does when you turn the key tells you where to look. If it cranks normally but never fires, the battery and starter are fine and the engine is...Read guide
Steering & ChassisWhat Does a Bad Wheel Bearing Sound Like?
A failing wheel bearing makes a humming, growling or droning noise that rises with road speed rather than with engine revs, and it is very often mistaken...Read guide
Steering & ChassisHow Do I Know If My Shock Absorbers Are Worn Out?
Worn shock absorbers let the car keep bouncing after a bump instead of settling immediately, because the shock's job is to control the spring rather than to...Read guide