So ive finally decided to take the plunge and get that quad tailpipe exhaust done for my E60 525i.

Ive checked real oem and im abit confused.

Im not sure whether or not the cat is on the headers itself or the box under the center of the car where my 2 pipes, one from each header, enters and exits as 1 lone pipe which is larger.

Ive already arranged to have the 2 pipes coming out from the headers straight all the way to the exhaust with no resonator/cat in between from the headers to the mufflers but someone told me that there is a sensor before and after the box in the midsection of the car and if you remove the box and run straight pipes all the way and use the sensors give the same reading before and after, i will get a check engine light.

Can anyone confirm this?

Thanks a bunch guys for the responses in advance.

 

The two sensors (four total?) are your oxygen sensors. And yes... if you remove the cat you could get the CEL.

http://auto.howstuffworks.com/question257.htm