Advanced emission (CAT) test:

CAT test is an unfortunate term which leads many to think that the MOT is testing the Catalytic converter fitted to later vehicles but this is not so.

The MOT is testing emissions, so if the emissions from the tailpipe meet the requirements the vehicle passes. Whether a CAT is fitted is irrelevant. The deciding factor is the vehicles date of first registration. A "Cat" may be fitted to a vehicle which only needs to meet the two gas test and if the emissions are satisfactory, it passes. A "Cat" may have been removed and the vehicle is required to meet the "Cat" test standard by virtue of its first registration date. If the emissions meet the standard then it passes.

Many advanced design engines will meet the "Cat" requirements with a standard exhaust but of course may be emitting other pollutants not subject to the MOT.

Advanced emission tests must be carried out on fully warmed up engines, the oil temperature must be above 80 degrees C (some are set at 60 degrees C) and the idle speed must be within the manufacturers agreed tolerances. Both engine speed and oil temperature will be measured using equipment attached to the gas analyser.

During engine idle the engine must not be subject to electrical loading such as heated seats, heated rear windows and air conditioners. Electric cooling fans do not cause significant problems and can be active during the test.

Twin exhaust systems need to have the reading from each pipe tested and then averaged.  A twin exhaust has two separate pipes from the engine manifold all the way back to the tailpipes. Systems with a balance tube between separate pipes are still considered a dual exhaust.

Multi fuel vehicles should be tested on the fuel they are running on when presented for test e.g a vehicle which runs on petrol and LPG and is running on LPG when it's presented, will be tested on LPG. The difficulty is that the hydrocarbons emitted by LPG vehicles are propane rather than hexane so the tester needs to apply a conversion known as the PEF factor, usually marked on the gas analyser, to give a sensible reading.

Where vehicles fail the advanced emission tests, remedial action should concentrate on fundamentals such as plugs, timing, air cleaners, mixture settings and the oxygen sensor before changing expensive catalytic converters. Leaks in the exhaust system are a common source of problems.