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Decatting a Diesel
You CAN remove the catalytic converter on your Diesel Car without having a problem with the MOT. Having taken advice from VOSA, who control MOT testing in the UK, we are 100% confident that removing the CAT on a Diesel won't cause an issue. The confusion has arisen due to the regulations changing in January 2012 for cars which qualify for a full gas emissions test where 'if a cat was fitted as standard, it must be there for the MOT'. As Diesel cars don't qualify for a full gas emissions test, the regulation doesn't apply. The same is true for DPFs which you can also remove (they're not even mentioned in the manual)
Anyone can download the MOT testers manual
HERE (you'll want the one for Class 3, 4, 5 and 7) and have a look. The part that deals with exhausts is Part 7.1 on page 157 (yes we've looked at this a lot!)
Obviously things can change but two inspectors from VOSA have both given the same information and said that nothing is going to change in the near future. If things do change then we'll be the first to let you know.
Information correct as of April 2012