Does a diesel car need a catalyser to pass mot emissions test?

Question

I have been told a emissions test for diesels in the UK only tests for smoke or particulates, Is this correct? I am tuning my 530d and want to improve performance by fitting a bypass instead of the catalyst, but still want it to pass the mot without the hassle of fitting it back for the mot every year.

Answer 1

Diesels don't use a catalyst in the manner that petrol engines do but there may be exhaust system components that reduce emissions on a diesel. If you modify the emissions control system in any way you'll fail the test. It is also illegal to operate a vehicle on public roads with an improperly modified emissions control system.

The catalyst-like component on a diesel is usually a specialised type of filtration system used to reduce particulate emissions. If it has clogged to the point that it affects performance or economy a simple replacement is in order and will return your car to the expected performance.

Answer 2

I am an Mot tester and have yet to fail a deisel unless it emits quite considerable smoke.
I would advise you to add a deisel treatment to your fuel and thrash the car to within an inch of it's life prior to the test if in any doubt.