My B pipe to cat flange finally disconnected from rust. Drove on it a while. Gas mileage went down. I "fixed' it with a clamp kit. There is still a little leak but it sounds good enough for winter.

I've noticed the car is posting -4 LTFT with -3 STFT at idle now. Every time I moved the CAT around to line it up with the B pipe flange I would get a P0430 and the mode6 test would be bad. Sometimes bumps would send STFT to -22 territory, etc.

If I drive on the highway, however the fuel trims, long and short return to 0. Since the junction is now secure and not flopping around the mileage went up too. Had to make several attempts to find a decent enough seal, each time moving the cat, each time code, more negative fuel trims or less. Driving on the highway the code actually went away and I had some really good test results.

So... Am I right to suspect the header to cat gasket/joint as leaking? The spark plugs do not exhibit any signs of running rich. I don't feel any air exiting around the connection. Will do a pressure test for sure if its warm again. Beyond that what could it be? Valve adjustment was done last year. Spark plugs are clean, no O2 sensor codes, etc.