Ignition or combustion misfire
Spark, fuel, compression or control faults can cause one or more cylinders to burn inconsistently.
Useful clues: Rough running, shaking, poor acceleration, fuel smell or a flashing warning light. Car problem guidesSYMPTOM AND DIAGNOSIS GUIDE
Check Engine Light On: What It Means and What to Do
The check-engine light means the vehicle has detected a fault that can affect emissions, engine control or related systems. It does not identify a part by itself. A diagnostic trouble code points to a monitored circuit or condition; testing is still needed before parts are replaced.
Check a likely repair cost →Published by David Burridge, ShiftyMechanics founder and editor. General consumer guidance reviewed . It does not replace inspection by a qualified repairer.
Possible causes and useful clues
The same symptom can have several causes. These clues help describe the fault; they do not prove which part has failed.
Air or fuel measurement fault
Air leaks, airflow sensors, oxygen sensors, fuel pressure or injector faults can upset the commanded mixture.
Useful clues: Poor fuel economy, hesitation, hard starting or lean/rich mixture codes.Evaporative-emissions leak
The fuel-vapour system may detect leakage from a loose cap, hose, valve or sealed component.
Useful clues: A steady light with little change in driving behaviour, sometimes appearing after refuelling.Catalyst or exhaust-system issue
A failed sensor, exhaust leak, persistent misfire or worn converter can trigger catalyst-efficiency monitoring.
Useful clues: Sulphur smell, rattling, reduced power, repeated oxygen-sensor or catalyst codes.Electrical, sensor or control problem
Damaged wiring, poor connections, low voltage or an implausible sensor signal can cause a warning without a mechanical component being directly at fault.
Useful clues: Intermittent light, multiple unrelated codes, recent battery trouble or faults that change with vibration or weather.Safe checks before the booking
Only carry out checks you can perform safely and within the owner's manual.
- Observe whether the light is steady or flashing and whether any red warning is also displayed.
- Note changes in idle, acceleration, fuel use, temperature, smoke, smell or starting.
- After switching off, confirm the fuel cap is correctly fitted if your vehicle uses a conventional removable cap.
- Record recent refuelling, battery replacement, repairs, water exposure or accessory installation.
- Do not erase codes before diagnosis; freeze-frame and readiness information can help reproduce the fault.
What a workshop should check
Ask what measurement or inspection supports the diagnosis.
- Read current, pending and history codes plus freeze-frame data
- Confirm the complaint and review live sensor data under relevant conditions
- Test the indicated circuit or system rather than treating the code description as a failed-part instruction
- Complete a road test and readiness check after repair where safe and appropriate
Questions to ask before approving work
A clear answer makes it easier to compare the scope of different quotes.
- What code and freeze-frame conditions were recorded?
- Which test confirmed the proposed part is faulty?
- Could wiring, leakage, low voltage or another fault produce the same code?
- Will the repair include clearing, road-testing and confirming the monitor completes without the fault returning?
Avoid these common mistakes until the cause has been properly checked.
- Buying the part named in a code description without testing the system.
- Ignoring a flashing warning light or severe misfire.
- Clearing codes repeatedly, which removes useful evidence and can hide an unresolved emissions fault temporarily.
Know which repair was recommended?
Select your vehicle and repair to review the available price reference, job scope and important limitations.
Open the repair-cost calculator