If i was to run a full decat system on my 05 STi and come MOT time i was to put the original Prodrive system back on but not have it remapped would it pass MOT? We talking having it back on just for the test and once home i will replace the decat system again.

 

should be ok to pass, the mot test should be done at closed loop anyway just seems a lot of hassle for what you will gain, I would rather run a sportcat than a decat.