98 Grand Prix engine problems

Yes it is I who is now working on engine #3. The engine went in fine for 24 hours and then the engine light came back on. I took it to the shop that guaranteed the labor they said I needed an oxygen sensor (again) and mass intake filter of some sort. That worked for almost 24 hours before the engine lite came back on again.

I'm out of ideas any suggestions? Keep in mind that I have a new radiator, water pump, hoses and I might add that my temperature gauge goes above 210 since the new motor. I don't want it to blow a rod like the last one did.

Could the catalytic converter make the car get hot? and aren't those connected to the oxygen sensors?
I don't know what to do from here help.......Thanks, Lori what are the codes and are you loosing coolant? no I am not losing cooliant. I just called the auto shop because they just fixed a oxgyen sensor and asked them what the code was and he said it was 420.
But the engine lite has come back on and they want me to bring it back again. The cooliant lite is off. any suggestions? should I take it back again?
Thanks, Lori Did you check the thermostat? It may be seized! the thermostat was replace 1 time with the engine that froze and again with the engine that blew.

I just got back from the auto shop and the code is: DTC P0420
thanks, Lori have the cat converter replaced,it has an 8 year 80000 mile warranty.I forget your mileage but it being a 98 may be in warranty