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Dreaded P0420 code. I've sprayed carb cleaner in the engine bay to test for air leaks, I've cleaned MAF sensor, replaced PC valve, no leaks in header or exhaust. Reading O2 sensor voltages produces these results. Under load both left and right post cat sensors read pretty similar. When coming off the gas both eventually drop to a lean condition. Then left post sensor almost immediately jumps to full rich. I don't think it’s a leaky injector because the left pre cat sensor doesn't spike. If anything it goes just a little lean on the left side (compared to right). I switched the post cat O2 sensors to see if it could be the sensor itself. No difference. I think it has to be a cat going (gone) bad. Does this all jive? Thanks a bunch for the help!!