If a car passes a BET test on the MOT we can safely assume that the cat - or what looks like a cat - is working.
Only if it fails the BET test do we continue on to do the full cat test. The full test includes measuring the engine oil temp to ensure that the engine is up to full operating temperature, and allow a longer warm up period for the cat.
Don't let's forget that the BET test is done to tighter limits than the full cat test. If we could all run cars without cats and still get below 0.2co and a lamda of 1 readings I think a lot more people would.
But to confirm that diesels do not require cats to be fitted for the MOT test
Nick
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