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If you have removed the front cat and not changed the ECU flash calibration then yes you'll get a check engine light as the O2 sensors in the exhaust will be showing the exhaust emissions are well out of range
so if this is a track car where you don't have to run the cat then you'll need an ECU calibration change to over come this
however a road car MUST have a cat or you're breaking the law and it certainly won't pass the MOT
can I ask why you have removed it?
if the front cat was clogged / broken up then this can really hurt engine power so you sometimes remove the cat as part of the diagnostic process
but you do need a cat
how about a good used item?
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