How many O2 sensors are there on the 2002 3.8 Impala's? I have replaced the O2 sensor behind the engine before the CAT. The mech reset the code. I saw him install the part. The check engine light is still on. The O'Reilly guy checked it for me and the same code is still tripped. (Bank 1, low thershold).

did you replaced the one that is on the exhuast manifold? I believe there is only two

Yes, the one behind the motor before CAT.

I talked to a service tech and he said that to make the light go away you will probably need to go OEM on the CAT. He said that he replaced the cAT adn exhuast and had to put an O2 simulator on the rear CAT. He said that it will tell the car the CAT is OK.

I have read up on the CAT stimulater. Not sure yet. Thanks for your info.