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.