Dutch is close enough to German that I understand his car won't pass emissions testing (MOT), and that his car has a catless Invidia downpipe. Here's my advice (in German, sorry).
Sie mussen das Fallrohr mit einem catted Fallrohr zu ersetzen. Invidia macht.
Sorry if I butchered that, my German is a bit rusty, and I needed Google translate to fill in a few words...
Basically, you probably need a catted downpipe, which Invidia makes. No guarantees though, as I'm not familiar with Dutch auto testing.
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