A fault code is a clue, not a diagnosis. Look the code up free, and if you're still stuck, send it to Bob — a qualified UK mechanic with 32 years on the tools — and get a written answer about your own car within 6 hours for £14.99. Money back if it doesn't help.
Why the code on its own hasn't fixed it
A code tells you which system the ECU is unhappy with — not which part has failed. P0420 does not prove your catalytic converter is dead. P0299 does not prove the turbo is finished. The same code can have half a dozen causes, and the expensive one is rarely the most common one.
What turns a code into an answer is context: your registration, the mileage, exactly when the problem happens, what the car feels like, and what has already been replaced. That is the part a code reader, a forum thread and a generic AI answer cannot do.
The most expensive mistake is replacing the part named in the code without checking the cause — a £900 catalytic converter for a P0420 that was actually a £40 exhaust leak. A £14.99 written answer before you authorise anything is the cheapest insurance there is.
Start here — it's free
Look the code up in the free fault code library for the plain-English meaning, common causes and UK cost bands. Match the symptom in the car problems library, and check the MOT history — advisories and past failures often point straight at the current fault.
If those leave you with two or three possibilities and you don't know which to check first, that is when a paid written answer earns its keep.
Frequently asked questions
I've read the fault code — why am I still stuck?
A fault code is a clue, not a diagnosis. The same code can be several different faults. Send the code, the reg and what the car is doing through Ask a Mechanic (£14.99) and Bob replies in writing within 6 hours with the likely causes in order, the cheapest check first, and a realistic UK repair cost.
A garage quoted me hundreds for a fault code — is that fair?
Sometimes yes, often no. The most common overcharge is replacing the part named in the code without checking the cause. Send the code, the quote and your registration and Bob will tell you whether the diagnosis fits and whether the price is normal for the UK.
Will a fault code fail my MOT?
The stored code itself does not fail the MOT, but an illuminated engine management light is an automatic major failure. Clearing the code the morning of the test does not work — it also resets the emissions readiness monitors. Fix the fault first, then book the test.
Is it safe to keep driving with a fault code stored?
A steady amber engine light with the car driving normally is usually safe to drive gently to a garage. Stop driving if the light is flashing, the car is in limp mode, there is heavy smoke, or the temperature gauge is climbing. Any red warning light means stop as soon as it is safe.