2001 Cat Converter - Toyota Corolla, Chevy Prizm (1998-2008)

Not too long ago I completed an engine swap on my 2001 corolla that had about 212000 Km on it.

The engine had run low or completely dry on oil a couple times damaging the engine, starting burning oil and eventually deteriorated the bearings. http://www.corolland...-engine-rattle/

A month before the engine suffered catastrophic failure, I replaced the post cat O2 sensor and it passed the emissions test. The engine failed so I swapped in one with 112000 Km.

Now I am getting a P0420 Cat converter code.

Is it possible that the cat converter is working fine one day, then after the engine swap it fails? OR is it more likely that it's the upstream O2 sensor that needs replacing?

A year before the engine failed, I had also gotten a MAF failure but cleaned it and the code went away.

I also read that an engine that burns oil can cause rapid failure of the cat, so...

Any thoughts?

The P0420 CEL is set by the downstream O2 sensor only, the upstream one is the main sensor for setting the air/fuel mix. In cases of heavy oil consumption, there could be chance that the catalytic converter is clogged/damaged by the oil in the exhaust. Given the amount of mileage you had on the car, 212000 KM, coupled with a oil consumption - strong likelyhood that the catalytic converter is bad. But in your case, there might be some life left in the cat, given that it didn't pop the code before.

What is more likely happening is there is an exhaust leak that is causing P0420 introduced post-swap. I'd make sure that none of the pipe unions are leaking, assuming you swapped in new gaskets and clamps for the exhaust where needed? Also depending on the swapped engine (model year, mileage, condition), could be a faulty sensor/miss timing that is causing the P0420 to pop up. Wouldn't be a bad idea to double check all electrical connections, make sure chassis grounds are in good conditions, check for obvious signs of damaged/pinched wiring, damaged/missing vacuum hoses. The only definite symptom here is that the problem showed up, immediately after the swap or atleast one drive cycle completed.