Nearly one in three UK cars fails its MOT every year — and a significant proportion of those failures are caused by OBD2 fault codes triggering the engine management light. This guide breaks down the fault codes most likely to cause a UK MOT failure, how serious each one is, and what it costs to fix before your test.
MOT failure due to OBD2 happens in two distinct ways:
This means some cars fail their MOT twice over from a single fault code — once for the warning light and once for the emissions test.
| Code | Description | MOT failure risk | Typical UK fix cost |
|---|---|---|---|
| P0420 | Cat efficiency low (bank 1) | Very high | £80–£800 |
| P0430 | Cat efficiency low (bank 2) | Very high | £80–£800 |
| P0171 | System too lean (bank 1) | High | £50–£400 |
| P0174 | System too lean (bank 2) | High | £50–£400 |
| P0300 | Random/multiple misfire | Very high | £60–£500 |
| P0301–P0306 | Cylinder-specific misfire | Very high | £60–£300 |
| P0401 | EGR flow insufficient | High (diesel) | £100–£600 |
| P2002 | DPF efficiency low | Very high (diesel) | £200–£1,500 |
| P0128 | Coolant below thermostat temp | Medium | £80–£200 |
| P0440–P0455 | EVAP system leak | Medium (light on) | £0–£300 |
| P0456/P0457 | EVAP small leak / loose cap | Low (often free fix) | £0–£50 |
| P0135/P0141 | O2 sensor heater fault | Medium | £80–£250 |
| P0340 | Camshaft position sensor | Medium | £100–£300 |
| P0700 | Transmission fault | Medium | £100–£800 |
P0420 is the single most common code to cause UK MOT failure. The catalytic converter is no longer processing exhaust gases efficiently — meaning the car both has its warning light illuminated AND fails the tailpipe emissions test. It's a double failure. The repair cost varies enormously depending on the root cause: sometimes it's a faulty oxygen sensor (£80–£150), sometimes it's the catalytic converter itself (£300–£800 for most UK cars). Getting a vehicle-specific diagnosis before visiting a garage is critical here — garages frequently quote for a new cat when a sensor is the actual cause.
Active misfires cause the check engine light to flash (not just illuminate) and produce high hydrocarbon emissions from unburnt fuel passing through the exhaust. This fails both the warning light check and the emissions test. The most common cause in UK cars is worn spark plugs (£60–£150 for a full set fitted) or a failed ignition coil (£80–£180 per coil). These are among the cheaper MOT failure repairs if caught early — but if left, the unburnt fuel destroys the catalytic converter, turning a £100 repair into a £500+ one.
A lean-running engine produces elevated NOx emissions and causes the warning light to illuminate — both MOT failures. Common causes include a vacuum leak in the intake system, a dirty or failing mass airflow (MAF) sensor, or a clogged fuel injector. A MAF sensor clean costs £30–£60 DIY. A replacement MAF sensor costs £80–£200 fitted. A vacuum leak can sometimes be fixed with £5 of silicone hose.
DPF (diesel particulate filter) faults are the diesel equivalent of catalytic converter faults — and they're increasingly common as the UK diesel fleet ages. A blocked or failing DPF causes excessive particulate emissions and triggers the warning light. Fix options range from a forced regeneration (£50–£100 at a garage) to a full DPF replacement (£500–£1,500). Regular motorway driving helps prevent DPF blockage; lots of short urban trips cause it.
EGR (exhaust gas recirculation) valve faults are extremely common on UK diesel cars, particularly those used mainly for short urban journeys. A clogged EGR valve causes elevated NOx emissions and illuminates the warning light. The EGR valve itself costs £100–£300 — but before replacing it, a professional clean of the EGR and inlet manifold (£100–£200) often resolves the fault for far less.
Before spending money on expensive parts, always check these first — they're the cheapest possible fixes for codes that cause MOT failure:
| Check first | Codes it may fix | Cost |
|---|---|---|
| Tighten or replace fuel cap | P0456, P0457 | £0–£15 |
| Replace spark plugs | P0300–P0306, P0316 | £20–£60 DIY |
| Clean MAF sensor | P0171, P0174, P0101 | £5–£15 DIY |
| Replace thermostat | P0128 | £15–£40 parts |
| Check/replace O2 sensor | P0420, P0135, P0141 | £30–£80 parts |
| DPF forced regeneration (motorway run) | P2002 | £0 (fuel only) |
The ideal time to scan your car for fault codes is at least 2–4 weeks before your MOT. This gives you time to:
The most expensive mistake UK drivers make is booking an MOT without knowing their car's fault code status. A £1.59 AI diagnostic report from AI-Diagnostics-Pro tells you exactly what each stored code means for your specific vehicle, the likely repair cost at a UK garage, and whether it will fail your MOT — giving you time to fix it before the test rather than paying a re-test fee on top.
Instant AI-powered report — UK repair costs, MOT impact and step-by-step fix guide for your exact vehicle.
Get My AI Diagnostic Report — £1.59 →Disclaimer: AI-Diagnostics-Pro provides information for educational purposes only. Repair costs are estimates based on UK market data and may vary. Always consult a qualified mechanic before carrying out vehicle repairs.