mechanics.trade
Car diagnostics and fault finding
A diagnostic check reads the fault codes a car stores when something is not behaving as expected.
What car diagnostics means
The code narrows down where to look; it does not name the broken part on its own. A good garage will follow it up with live readings and a road test before recommending anything.
What the job covers
- Full system diagnostic scan
- Engine management light investigation
- Live data and road testing
- Electrical fault tracing
- Written report of faults found
When you might need it
- The engine management light is on, flashing or coming on and off.
- The car goes into a reduced power mode, sometimes called limp home.
- Fuel economy has dropped, or the engine idles roughly, with no obvious cause.
What it costs, and what changes the price
Typically £40 to £120.
- Most garages charge a set fee for the initial scan and road test.
- Many will put that fee towards the repair if you have the work done with them, which is worth asking about.
- Some faults need hours of testing rather than minutes, and that is charged as labour.
How long it takes
A scan and road test is usually under an hour. Chasing an intermittent fault takes longer.
What the MOT data shows
Out of 42,728,066 MOT tests in 2025, these are the parts covered by this job that testers recorded as faults.
| Part | Recorded in 2025 |
|---|---|
| Malfunction indicator lamp | 535,874 |
Common questions
Does a code tell you exactly what is broken?
No, and this is the most useful thing to know about diagnostics. A code for a lean mixture, for example, can be a leak, a sensor or a fuel supply problem. The code says where to start looking.
Can I just clear the light?
You can, but if the cause is still there the light comes back, and clearing it wipes the stored information a garage would have used.
Will the engine light fail an MOT?
The malfunction indicator lamp staying on is a failure item, and it was recorded over half a million times in 2025.