[jamie1983]

Had BMW performance exhaust installed and secondary cats gutted, afterward the "service engine soon" light came on.

Coincidence or related? Is it necessary to have the post cat o2 sensor?

[STR8-6IX]

Originally Posted by jamie1983

Had BMW performance exhaust installed and secondary cats gutted, afterward the "service engine soon" light came on.

Coincidence or related? Is it necessary to have the post cat o2 sensor?

is it necessary to have the post cat o2 sensor? is that seriously a question? of course it is.. and Im willing to put money on that fact this is exactly why you have a check engine light.

as for your secondary cats, they do not have their own set of o2 sensors so this does not interfere with emissions or cause a CEL in any way (I've tried it myself)