2000 Isuzu Rodeo code PO420

My 2000 Isuzu Rodeo had a blown engine and had the engine replaced with a used engine with 40,000 miles on it. I had the engine put in by a bad mechanic. When I picked up my vehicle the check engine light not only stayed on, but it blinked when I excellerated. It also idled roughly and drove roughly. I got a scan tool and it showed code PO300. After checking about everything I could I reluctantly took it to a dealer. The dealer mechanic said that the O2 sensors were plugged into the wrong banks causing the misfires. The code was reset and I passed emissions. When I drove the vehicle later that night the check engine light came on again. My scan tool showed PO420. I erased the code and for a day now it has not came back on. Any suggestions as to the cause of the check engine light returning after the O2 sensors were switched properly.

exhaust leak in the manifolds from the engine job maybe!

I am a little slow in the mechanic area so what should i do to check

have it checked by either the mechanic that installed the engine or the dealer that switched the 02's. If the exhaust system has a leak ahead of an 02 it can cause that code. So can a failled Cnnverter