Common causes and typical repair costs
A diagnostic scan is the starting point, usually £40 to £90 and often refunded against the repair. These are the causes garages see most.
| Job | Typical UK cost |
|---|---|
| Diagnostic scan and report | £40 to £90 |
| Faulty lambda or oxygen sensor | £120 to £300 |
| Ignition coil pack or spark plugs | £120 to £400 |
| Mass airflow sensor | £130 to £300 |
| EGR valve clean or replacement | £200 to £700 |
| Boost or MAP sensor | £120 to £280 |
| Catalytic converter | £250 to £1,500 |
| Injector replacement (each) | £200 to £500 |
What affects the cost
Steady or flashing
A steady amber light means get it looked at soon. A flashing light usually indicates a misfire that can destroy the catalytic converter, so stop driving and get it recovered.
Codes are a clue, not a diagnosis
A code pointing at a lambda sensor does not always mean the sensor is faulty; it may be reporting a genuine mixture problem. Paying for proper diagnosis beats replacing parts on guesswork.
Limp mode
If power is sharply reduced, the ECU has protected the engine. That points to something more serious, commonly boost, fuelling or a sensor the engine cannot run safely without.
Cheap code readers
A £20 reader tells you the code but not the cause. Garages use live data, freeze frames and manufacturer-specific tests, which is what you are paying for.
What to do when the light comes on
- Check the fuel cap is tight, a genuinely common cause of an evap code.
- Note whether the light is steady or flashing, and whether power feels reduced.
- Check for obvious signs: smoke, noise, temperature gauge, warning messages.
- If the light is flashing or the car is in limp mode, stop and get it recovered.
- Book a diagnostic scan promptly rather than waiting for the next service.
How to pay a fair price
Ask garages whether the diagnostic fee is refunded against any repair, and get the fault codes and their findings in writing. With that in hand you can compare repair quotes on the same diagnosis rather than paying twice to have the fault found.
Compare local quotesFrequently asked questions
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