I have a 2002 BMW M3, and I've been trying to track down a poor fuel economy issue for quite some time now. My long term fuel trims are high (+7%-10%), and when I had the car on the dyno a couple months ago my AFR was very rich at the top end (10-11). The ECU has no trouble codes. I've looked at every other possible cause (intake leaks, fuel system etc.) with no luck and I'm wondering at this point if it could be that my cats are the problem. (a bit clogged up?)

I've read that one way of knowing that you have bad cats is to look at the post-cat O2 sensor voltages -- they should stay constant around 0.8V whereas the primary O2 voltages oscillate. Well, under most conditions, my post-cat O2 sensors are constant-ish, but in certain partial throttle conditions they will oscillate (like the primaries) for a few oscillations before stabilizing again. I'm wondering if this is enough to point to the cats as my problem, or is this transient oscillatory behavior normal? It would be great if I didn't have to do a backpressure test to know for sure that my cats are bad -- the primary O2 bungs are a frickin' pain to get to on the S54 and at that point I might as well just swap the headers/cats out with a new (used) set.

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