I just replaced my muffler. In the process I = removed the catalytic pipe and disconnected my oxygen sensor plug. I cleaned up = the various joint services and installed new gaskets and bolts and put = everything back together. I did not remove or mess with the oxy sensor itself. = other than looking at it inside the cat pipe. I plugged the oxygen sensor jack = back in and went for a test drive. initially it ran fine ( I guess that is = closed end loop) but when the engine started to warm up then my performance went = to hell. I disconnected the oxygen sensor plug and could drive home with good = engine performance. previously a mechanic had told me that my oxygen sensor = is getting lazy and that I should replace it soon. I am planning on doing = that soon but in the interim I can't figure out why a new muffler install = would cause such problems with the oxy sensor. it was working relatively = fine right before the new muffler.