I am looking to put flowmaster"s American Thunder exhaust system on my 08 f150. I have some questions and concerns. The systems comes with muffler, and cat back pipes. The new muffler ties into the downpipe. Will I affect my O2 sensors or cause my check engine light to come on or fail admissions.

 

Nope, as long as all mods are done after the downstream O2's.

 

What do you mean it ties into the downpipes, does it replace the Y pipe? Typical cat-backs only replace the system from after the original Y pipe. There is an immediate connection joint on the Y pipe outlet. This would not affect anything.

Now if you are replacing the Y pipe with another catted one, nothing will be negatively affected, but an O2 sensor may trip a CEL if it gets touched on removal. If you are replacing with a non catted Y pipe you will get the same CEL for a catalyst inefficiency reading.