Car diagnosis by symptom: possible causes and OBD codes

Pick what you notice and when it happens: see possible causes, related OBD codes and what to check before replacing parts. Indicative results.

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How diagnosis by symptom works

Choose what you notice and when it happens. You see possible causes, related OBD codes and what to check before replacing parts. Results are indicative, not a confirmed diagnosis.

Symptom guides

Car losing power: causes and what to check — Reduced power can come from the engine or transmission. A protection mode is a response to a problem, not a diagnosis.
Car won't start: causes and step-by-step checks — First distinguish no cranking from an engine that cranks but does not start. Sound alone does not confirm a failed part.
Dashboard warning lights: what each one means and what to do — A guide to 16 common dashboard lights: how to recognise them, what they can mean and when to stop safely.
Car hard to start when cold: causes and checks — Separate an engine that does not crank or cranks slowly from one that cranks normally but will not start. Record ambient temperature and parking duration: a cold engine…
Diesel shaking at idle when cold: causes and checks — Cold-start vibration may result from irregular combustion, fuel delivery or engine mounts. Distinguish unstable RPM from vibration at steady RPM.
Engine overheating: what to do now and the causes — A temperature warning requires stopping safely. A correct coolant level does not rule out a cooling fault.
White exhaust smoke: what it means and when it is serious — Vapour that quickly disappears after a cold start can be condensation. Persistent white smoke, especially with coolant loss, needs investigation.
Why does the car jerk when accelerating? Causes and checks — Record whether interruptions occur under acceleration, at steady speed or exactly during a gear change. These situations can call for different engine and transmission…
Why does the engine shake at idle? Causes and checks — Unstable speed or rough idling can involve ignition, fuel delivery or intake faults. Petrol and diesel checks differ.
Why is my car blowing black smoke? Diesel and petrol causes — Black smoke can indicate incomplete combustion: too much fuel relative to air. The fuel and intake systems need checking.
Why is there blue smoke from the exhaust? Causes and checks — Blue or bluish exhaust smoke can indicate oil burning inside the engine. The cause needs checking; smoke colour alone does not identify a failed part.
Yellow coil light on while driving a diesel: what it means — The spiral indicator can mean preheating or, on some vehicles, an engine-management fault. A flashing lamp while driving has manufacturer-specific meanings.

Dashboard warning lights

Analyse several OBD codes together · AI second opinion