[duffyatkinson]

Originally Posted by Mike McKinney

Thanks for the reply, from reading your posts, your knowledge is exactly what I need. Here are my answers:

1. When I spoke with the MF tech on the phone I passed on the model numbers and he stated that they were correct and shouldn't have caused a problem.

2. The flow issue comes from talking with several folks, the MF guy, and a couple of exhaust folks. It's not a definite answer by any means but that's what most of them said, especially because they said the OEM cats are so efficient and the tolerance with other cats is small.

3. I've only changed out one rear sensor with no luck. After reading one of your posts that may be my next step, just change out all four.

4. I had it checked over for leaks once and he did find a few minor ones but haven't checked since.

I realize that there still could be an issue with leaks, bad sensors, wrong cats, but don't know how to definitively troubleshoot it without spending a fortune. It's also a fact of finding an exhaust specialist that I trust to really give it a good look-see. Thanks in advance for anymore help you can give me. I'm really hoping that it's a simple fix and I won't have to change out the cats or something crazy like that.

This is one of those situations that arise when you go so far away from stock that it is damn near impossible to reduce a symptom to a single cause. Is it the new cat(s)? Are the sensors sending data to the ECU that is outside the fuel map? Is it just a wierd leak somewhere?

Based on your narrative, you are receiving both the 4X0 codes, which means that the ECU is reporting combustion byproducts transiting BOTH the cats outside of acceptable ranges. Either both of the cats are structurally failing, unable to process the combustion stream sufficiently for the ECU, or the cause is upstream from the cats, i.e. header seals and/or the header-to-pipe joints.

See what I mean? It's gonna be tough to give you much help from here. If the answer isn't in any of the above places, then getting the exhaust system back to stock (painful, I know, but...) might be the only path you can take.

Good luck. Another example of why so many of us on here preach the benefits of keeping the ECU-related components stock.