This lady brought me this car and said her brother put the theromostat in backward and it needed a head gasket, GM 2. 2. I did the job and even had the head rebuilt. About 10 days later she had the car towed back to my shop said it would not run. I check it and one of the coil packs was not working so I replaced it and cleared the codes. 2 days later she says the check engine light is on again this time it is a P0420 (Catalyst system efficiency Low), I cleared the code again just to make sure and it came back the next day. So told her the misfire probably dumped fuel into the cat and ruined it. So I replace the cat and the next day same code came back. Then I looked at the oxygen sensors. Both are working and seem to be working fine. But I replaced the down stream one just in case. I drove the car a whole day no codes. Next day code came back. I ran the code through alldata and all it says is, "bad cat" or "exhaust system leak" I can find no leak. Anyone have any ideas.