This CEL is gettin me really pissed off mainly because any other problems are hindered when my p0420 CEL comes on. The CEL will come on for 2 days and go away for 2 days. For example, I'll get the tighten gas cap light but once my p0420 CEL comes on, the tighten cap light will never come on again until the CEL goes away. I was thinkin would it be possible to buy a used catalytic converter off someone from the black market, swap and would eliminate the CEL?

 

First step replace the rear downstream O2 sensor, if still having the issue it's the rear cat (one on the back of the block).