Hmm... normally that happens when the sensor is plugged in!
This is the problem with being a trail blazer kev
Next step imo is but the CAT back in and see if that solves all the problems. If it does, thats your answer, if you still have running issues then check the wiring going to the back of the precat is connected right and no breaks, if thats ok then change the pre cat lamda.
TBH its probably just the precat lamda is wither broken or the connection on the back of it, I am jsut suggesting the things to try that are free first.