I have 01 gt fed spec with cmf headers, highflow cat, and exhaust, and intake. I am currently running codes p0421 and p0431 something about it being below threshold. I do not have 02 sims .... is this the problem?
I know little about fed specs. But if you have a rear/secondary/downstream sensor, then yes, you would need a mecahnical O2 sim (spark plug nonfouler) in order to make the rear O2 sensor think that your cat is doing its job properly. The fouler pulls the O2 out of the exhaust stream a bit and makes the sensor think the cat is actually cleaning the exhaust (more than it is) before the exhaust gases reach the downstream O2.