ImpalaWhile undergoing many facelifts in its history, the Impala has proved itself to both civilians and police forces as one of the most capable 4-door cars GM has ever offered. Currently moving towards its 10th generation the Impala is one of the longest lasting and popular models.
About 2 months ago my Check Engine light came on so I took it to Advanced Auto for the free OBDII test and it came back with a code showing I had a bad Catalytic Converter. I immediately bought a direct fit Converter which costs me about $249.00 and had my mechanic put it on. He then reset the computer and the Check Engine light was now off.
One month passes and all of a sudden the Check Engine light comes back on again. I take it to Advanced Auto once more and they ran the test again but this time it came up with P0140 (Bank1, Sensor2). This means of course that the rear O2 Sensor is now bad and I proceed to go buy a new O2 sensor and have my mechanic take the bad one off and replace it with the new. He then resets that computer again and everything is fine until I drive down the road and 15 minutes later the Check Engine light pops back on AGAIN
I call my mechanic up and tell him the light is back on and he tells me to drive it awhile and it should go back off. Well, I did this and it still remained on. I then went back to Advanced Auto, had them run the test again, and I be damn, the code came back with P0104AGAIN! 
What could cause it to do this? I thought to myself, "Maybe it might be the front O2 Sensor which is causing it to give a false reading as the back sensor. So, I wait one week till I have enough cash and go buy the front O2 Sensor and I just had my mechanic take the old off and replace with the new. He reset the computer and I drove down the road and everything looked fine until about 15 minutes later and POW the Check Engine light popped back on. I am probably certain that if I check the code it will come back with the same P0140.
Can anyone help on this problem I am encountering? considering that I have a new Catalytic Converter, 2 new O2 Sensors, and a new inspection due on my car in 2 weeks. I need to get this fixed. Why does it keep popping back the same code? My only alternative I guess is to have my mechanic take that back sensor off and take it back and get another new sensor. Maybe the one I got was defective?
Any suggestions what else could be causing this? Hopefully I will get a timely response.
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Probably not the response you're looking for, but I would take it to the dealer. Or at least call them and see if they've heard of this before. I think it's the cat itself. Is it an aftermarket cat? I'm having the same problem with my acura right now. I bought an aftermarket cat and everytime my CEL comes on, the dealer pulls codes for the converter. I know your's is brand new but if it's aftermarket it could be the problem.
If you dont replace the cat with a stock cat you WILL get a code for the impale since our impalas are ultra sensitive. From what I gather u replaced it w/ an aftermarket cat therefore a code will for sure come on, the cheapest thing to do is to put in an o2 simulator if your car is made bet 00-03, this isimulator will always send the correct signal and won't really monitor the cat the link is below, otherwise you got to replace the cat with a stock.