On petrol cars, an emissions fail almost always means lambda sensor, catalytic converter, or a misfire you didn't know about. On diesels, it's usually DPF clogged, EGR sticking, or the AdBlue system on newer cars. A diagnostic scan before throwing parts at it saves a fortune.
Emissions failures are one of the most expensive MOT fails you can get — but they're also one where rushing into the wrong fix can cost you four figures unnecessarily.
Petrol — the usual suspects
High CO at idle: lazy upstream lambda sensor (£80-£150 fitted) is the cheap fix; cat converter failing is the expensive one (£300-£900). High hydrocarbons: misfire, faulty coil or plug — under £150 to fix. If your engine management light is on at the same time, get it scanned first because the stored code points straight at the problem most of the time.
Diesel — the usual suspects
High smoke value: DPF partially blocked (regen cycle needed, sometimes a forced regen at a workshop, occasionally a clean), or an EGR valve stuck open. Newer diesels (post-2015) can fail on NOx if the AdBlue system isn't dosing correctly. AdBlue faults are common — usually a sensor or a dosing module, often £400-£900 to fix.
What NOT to do
Do not let anyone 'remove' or 'delete' your DPF, EGR or AdBlue system to pass MOT. It's illegal since 2014 — testers are specifically trained to spot it, and your car will fail visual inspection. We get cars in every month that have been DPF-deleted by some shed and need the lot putting back on at huge expense.
The 'pre-MOT' fix that works
Before retesting, drive the car hard for 20-30 minutes on a dual carriageway — get it properly hot. On diesels this can trigger a DPF regen and bring emissions back into spec. On petrols it helps clear any moisture from the cat. We genuinely retest cars after a hot run and see them pass, no parts needed.
How much does an emissions fault diagnosis cost? — £45-£75 in most independents — a proper scan plus a smoke or gas test if relevant. Worth every penny before you start changing parts.
Can I just drive it like that until the next MOT? — No — once a car has failed MOT for emissions you can only drive it home or to a garage to be repaired. Driving on a failed certificate invalidates your insurance.
Will a retest cost the full MOT price again? — If the retest is at the same garage within 10 working days and the car only needed an emissions retest (no major component swap), it's usually free or a few quid. After that, it's a full fee again.
After an emissions failure, you have 10 working days to get the car repaired and back to the same centre for a free or reduced partial retest. Start cheap: an oil and filter change, a good motorway run and a fuel system clean fix many borderline diesels for under £150.
Answered by Bob — 32 years on the tools, DVSA-approved MOT centre, Birnam, Perthshire.
Car failed MOT on emissions — what can I do?
First, read the failure line on the MOT certificate. It will say whether the problem was carbon monoxide (CO), hydrocarbons (HC), lambda on a petrol car, or smoke opacity on a diesel. That tells a mechanic where to start, rather than guessing.
Second, check for an engine management light and read the fault codes. A failed oxygen sensor, misfire, leaking exhaust or blocked EGR valve is behind a lot of emissions failures, and the codes usually point straight at it.
Third, remember you can't drive a car with an expired MOT except to a pre-booked test or repair. If your old MOT is still valid you can drive it, but get the fault fixed before it runs out.
What an emissions failure usually costs to fix
Typical independent garage prices: an oxygen (lambda) sensor £90 to £250, spark plugs and coil packs £80 to £300, an exhaust leak repair £60 to £250, an EGR valve clean or replacement £150 to £500, and a catalytic converter £350 to £1,200. Retests at the same garage within 10 working days are often free or half price.
Frequently asked questions
My car failed the MOT on emissions — what should I do first?
Check the exact failure reason on the certificate (CO, HC, lambda or smoke), then have the fault codes read. Fix the cause the codes and readings point to, then book the retest within 10 working days if you can.
How much does it cost to fix a car that failed the MOT on emissions?
Often £90 to £300 for a sensor, plugs or an exhaust leak. A catalytic converter is the expensive end at £350 to £1,200.