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.