A code points at a circuit, not at a part
P0171 doesn’t mean “buy a sensor”. It means the engine is running lean — which could be a split hose, low fuel pressure, a dirty air flow meter or an exhaust leak upstream. Free code reads exist to sell parts. This exists to fix cars.
Don’t clear the code before you come. Freeze-frame data — engine temperature, load, speed and fuel trims at the moment the fault set — is usually the most useful evidence we have. Clearing it throws it away.
Ninety minutes, six steps
- Full-system scan. Every module, not just the engine — ABS, gearbox, airbag, body.
- Freeze-frame and fault history read and interpreted.
- Live data under load. Fuel trims and misfire counters tell a story a static scan can’t.
- Oscilloscope where a sensor signal or an intermittent fault needs it.
- Road test with the tool logging if the fault only shows at speed.
- Written finding with photographs — yours to keep whether or not we do the work.
What it costs
| Type | Price inc. VAT |
|---|---|
| Standard diagnostic investigation | £59 |
| Extended — scope work and road test logging | £129 |
| Electrical fault tracing, per hour after the first | £69 |
| Cooling system pressure test | £49 |
| Compression and leak-down test | £95 |
| Second opinion on another garage’s quote | £59 |
Credited in full against the repair if you go ahead with us.
If we can’t find it, we’ll say so. Some intermittent faults need a logger left on the car for a few days. You’ll be told what we ruled out, what it cost, and what the next step would be — we won’t start fitting parts to keep the invoice moving.
Second opinions welcome. Plenty of people come here with a quote from somewhere else and want it checked. £59 and you get our findings in writing, whatever they say.
Diagnosis first, then the repair page
Why pay when code reading is free?
Because they’re different jobs. The free read names a code and the counter names a part. If the part is wrong you’ve spent more than the diagnosis would have cost and still have the fault.
Should I clear the code first?
No — you erase the freeze-frame data and reset the readiness monitors.
Do I get the fee back?
Credited in full against the repair. If you don’t go ahead, you keep the written findings and can take them anywhere.
Will a warning light fail my MOT?
Airbag, ABS, brake and engine management lights are failures in their own right — so it’s worth sorting before the test, not after. MOT details.