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.
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.
Bob's Mechanical Repairs — independent family-run garage in Birnam, Dunkeld, Perthshire. Call 01350 727 276 or email [email protected].