#diagnosis
36 posts in Car Diagnostics blog about "diagnosis" — OBD-II diagnostics, DTC codes, repair. ← All posts
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ABS light after a wheel change: what you disturbed
The lamp appeared right after the tyre fitter finished. That timing is not a coincidence, and the cause is usually something knocked rather than something failed.
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Automatic headlights stopped working: sensor or setting
Before assuming a fault, check the switch position and what is stuck to the windscreen. Both explain more of these than any failed component.
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Bang on lift-off: exhaust backfire and what it means
A pop from the exhaust when you lift off is either an aftermarket calibration doing it deliberately or unburnt fuel reaching a hot catalyst. Those are very different.
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Belt squeal on a cold start: tensioner, pulley or contamination
A few seconds of squeal on a damp morning is a warning rather than an emergency — but the belt drives the water pump, and that changes the stakes.
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Heater warm on one side only: blend door or airlock
Hot air from one vent and cold from another means a flap is stuck or the matrix is not getting full flow. Two causes, two very different repairs.
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Alarm triggering by itself: how to find which sensor did it
Most alarm systems record what triggered them. Reading that record turns a neighbourhood nuisance into a specific repair.
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Starts then dies within seconds: immobiliser, fuel or idle control
An engine that fires and stops within two seconds is telling you something specific, and the timing narrows the cause more than any code.
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Will not rev past 3000 rpm: limp mode with a specific cause
A hard ceiling on engine speed is not the engine running out of breath. It is the ECU imposing a limit, and the reason is stored in the code memory.
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Clunk when you brake: caliper hardware, mounts or a worn joint
A single knock as you apply the brakes, and another as you release. That pattern means something is moving in its mounting when the load reverses.
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Clutch judder from a standstill: what it is telling you
A shudder as you pull away means the clutch is not engaging evenly. The cause is sometimes the clutch and often something around it.
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Creaking when turning at walking pace: bushes, strut mount or joint
A creak that appears at parking speeds and vanishes above walking pace is suspension rubber protesting, and locating it needs only a helper and a torch.
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Dashboard lights flickering: earth, alternator or module
Flickering instruments are a supply problem until proven otherwise, and the cheapest possible cause is the one to check first.
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Engine keeps running after you switch off: what causes run-on
An engine that shudders on for a few seconds after the key is out was a common fault decades ago. On a modern car it means something quite different.
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Exhaust smell inside the cabin: treat it as urgent
Exhaust gas contains carbon monoxide, which is colourless, odourless and disabling. Smelling exhaust in the cabin means something is letting it in.
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Fuel flap stuck shut: the release you did not know about
Standing at a pump with a flap that will not open is a bad moment to start learning. Most vehicles have a manual override, and it is worth finding now.
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Crunch on downshift: synchros, clutch drag or oil
A crunch going into a lower gear usually means the clutch is not fully releasing, not that the gearbox is worn out. The distinction is worth several hundred.
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Handbrake warning stays lit: switch, fluid level or EPB
The same lamp reports three different things depending on the car, and one of them is a brake fluid level you should check before driving.
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Hard to start when hot: the fault that hides on a cold morning
A car that starts perfectly cold and struggles when hot has a specific set of causes, and heat is doing the diagnosis for you.
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Hissing from the vents: blend door, refrigerant or a blocked drain
Some hissing from the dashboard is normal air conditioning behaviour. Some is a leak. The timing and the accompanying smell separate them.
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Humming that rises with speed: bearing, tyre or driveline
Three causes, and a free road test that separates them. Doing that test first stops people buying tyres for a bearing problem.
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Long crank on the first start: pressure that leaked away overnight
Three seconds of cranking on the first start and instant restarts all day. That pattern points at fuel pressure, and there is a two-minute test for it.
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Rain sensing wipers not reacting: the sensor lives on the glass
The sensor does not see rain. It measures how light reflects inside the windscreen, which explains almost everything about how it fails.
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Speedometer stopped: sensor, cluster or bus
A dead speedometer takes cruise control, the gearbox and sometimes ABS with it. What else stopped working tells you where the fault is.
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"Steering assist reduced": voltage before you condemn the rack
Electric power steering draws heavy current. That is why a message about the steering is so often a message about the battery.
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Steering wheel off centre after alignment: what went wrong
A wheel that sits crooked while the car tracks straight is not cosmetic. It means the alignment was done incorrectly, and it affects the safety systems.
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Surging at a constant speed: fuel trim, torque converter or EGR
A gentle rhythmic pulsing at a steady cruise, with your foot still. The frequency of the pulse points at which system is hunting.
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Sweet smell inside the car: coolant, and where it leaks from
A sweet, slightly syrupy smell in the cabin is coolant, and the most likely source is a component behind the dashboard that is expensive to reach.
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Ticking at idle: injectors, lifters or an exhaust leak
Some ticking is entirely normal and some means an engine is running out of oil pressure. The difference is in how the noise responds to temperature and revs.
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Traction control cutting in on dry roads: a sensor, not the weather
The lamp flickers and the engine cuts power on a dry straight road. The system is not malfunctioning — it is reacting to data that is wrong.
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Inner edge tyre wear: geometry, not the tyres
Tyres worn to the cord on the inside while the outside looks new. The tyres are fine — something is holding the wheel at the wrong angle.
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Vibration at idle: mounts, misfire or a dirty throttle body
A car that shakes at a standstill and smooths out when moving has a short list of causes, and one live-data reading tells you which.
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Washer pump silent: fuse, pump or a frozen line
A trivial system until you need it on a salted motorway. The causes are cheap and the checks take minutes.
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Water in the spare wheel well: seals, lights and drains
The lowest point in the boot collects everything that gets in. Finding the entry point is a systematic job, and the well itself is rarely the problem.
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Whistling from the engine bay: leak, bearing or belt
Three very different faults produce a whistle, and they are distinguishable by when the noise appears. That timing is most of the diagnosis.
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Windows fogging on the inside: where the water is coming from
Condensation on the inside means moisture in the cabin. Some of it is you breathing; some of it is a leak that will eventually cost real money.
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Wipers stop mid-screen: park switch and linkage
Blades that stop halfway up the glass instead of returning to the bottom point at one specific mechanism, and the fix is usually cheaper than the motor.