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Catalytic convertors are designed to lower the emissions of the car. A car doesn't need a cat for the MOT, but its very unlikely that the emissions would be low enough to pass the emissoins test without one.

As far as I'm aware, at least on older diesels, there is no such thing as a Cat. its a particle filter. Different kettle of fish.
 

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Decatting a diesel works very well.

There are two cats on the 530d, a primary cat and a centre.

Removing the primary cat (front cat) and replacing it with a straight pipe free's up the back pressure. This makes the turbo spool up faster and lower in the rpm range.

Only Petrol cars need a cat to pass emissions, diesel just use a smoke test at MOT time.
 
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