Why cars fail their MOT on emissions and leaks
Noise, emissions and leaks fail 3.03 percent of car MOTs. Often a cheap sensor, sometimes a catalytic converter or DPF. How to tell which on a salvage buy.
MOT DATA · 4 min read
A mixed bag of cheap and dear
Noise, emissions and leaks fail 3.03 percent of car MOTs. This category is a real mixed bag: some failures are a cheap sensor or a car that just needed a good run and a service, others are an expensive catalytic converter, diesel particulate filter or exhaust.
What fails
Petrol cars fail on high CO or hydrocarbons, often from a tired lambda sensor, a fouled cat, or simply short-trip use. Diesels fail on smoke and on the particulate filter, which is one of the pricier fixes. Oil and fluid leaks and a blowing exhaust also land in this category.
What it costs
A lambda sensor or a service is cheap and often clears a marginal fail. A blowing exhaust section is modest. The ones that hurt are a catalytic converter, which can run to hundreds, and a blocked DPF on a diesel, which is a very big bill if it needs replacing rather than regenerating.
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The salvage angle
On a salvage or MOT-failure diesel, treat an emissions or DPF fail with caution, because it can be the most expensive thing on the sheet. On a petrol car it is usually cheap. Either way, a car that has stood after an accident may fail emissions simply from sitting and short runs, so a proper drive can change the picture. The Analyzer weights diesel emissions risk higher than petrol.
Where to get the parts
Sourcing the part yourself rather than leaving it to a garage's mark-up is usually where the saving is. GSF Car Parts carries most of what a job like this needs, with click and collect from local branches.
Browse parts at GSF Car Parts →
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