Originally Posted by leoferus

If I was your technician I would have asked you to sit down to chat about what lead to the demise of the engine you wanted to replace. Without this information we would be skipping a very important part of the repair process. Then, I'd perform a health check with a Techstream. After that, I'd check your catalytic converter by performing an active test with which I'd force your fuel trim to go rich/lean and check to see how the sensors are reading those conditions.

If there's an obvious exhaust leak, well, I'd investigate it. Think about it for a minute.... where does an exhaust leak need to be to impact the oxygen charge enough that the sensors pick up the reading? Here's a hint, a muffler leak won't cause a P0420/P0430.

If I had to suspect the engine swap process I'd look for a large exhaust leak near the catalytic converter, sensors, etc. I'm speculating but speaking from experience, this seems like a classic case of a condition that existed prior to the last service performed that has manifested itself after service (service, in this case, being the engine swap). Refer to my opening sentence. I have enough experience to warn my customers that if they do not tell me the whole truth they can be looking at a lot more money being needed for repairs. If the head gasket failed, if bank one was dumping gas into the exhaust due to a stuck injector, leaking head gasket, cracked water jacket, etc, those could all cause the catalytic converter to fail.

Good luck.