Need help with 0420 and 0128 code!!

[Coldwaves]

Hey guys I've bee having some trouble with the P0420 code (catalysts system efficiency below threshold bank 1.) I purchased a blue driver code reader/scanner to monitor the live data of the O2 sensors and they both seem to be working fine. According to my research o2 sensors should give voltage readings of 0 to 0.9 which I am getting according to the live data info. The Cat converter was replaced with a new one from the dealer about 2 yrs ago so I highly doubt its a bad cat. However i did research that the P0128 code (ECT sensor) can throw a P0428 code. Som my question I guess is how do you properly diagnose a 0420 code and 02 sensors for proper operation?
Any and all suggestions would be greatly appreciated. Thanks

Regarding the P0420 code, I think the car computer compares the signals from the upstream and downstream O2 sensor to determine if the catalytic converter is effective. If the catalytic converter is working properly, the upstream O2 sensor should give readings that fluctuate and the downstream sensor should give readings fairly constant. This is because the car computer is constantly adjusting the air-fuel ratio, which leads to a fluctuating O2 from the exhaust. But the oxygen will be consumed in the catalytic converter when NOx and CO are burnt. I had a P0420 a few years ago after a misfire and I asked the shop to show me the O2 sensor reading from a GM Tech 2 reader. SO I would think a P0420 code actually point to a failed catalytic converter.

[mcdavidhome]

Ok thanks. I think when I looked at the live data I did see the upstream sensor readings fluctuate but I also think the downstream were fluctuating also but not as much as the upstream o2. Its just hard to believe that my CAT is bad again becuz it was just replaced by the dealer under warranty about 2 years ago. But thanks for the info I will go back and compare the reading and send a screen shot if possible.

[Coldwaves]

Ok thanks. I think when I looked at the live data I did see the upstream sensor readings fluctuate but I also think the downstream were fluctuating also but not as much as the upstream o2. Its just hard to believe that my CAT is bad again becuz it was just replaced by the dealer under warranty about 2 years ago. But thanks for the info I will go back and compare the reading and send a screen shot if possible.

Yeah, that seems to be pointing to the catalytic converter. I just think the catalytic converter in the Grand Prix is not that long lasting. For the fours years I owned my 2005 Grand Prix base model, I had two catalytic converter broken. At least a bad catalytic converter, unless it is clogged, is not going to influence how the car drives. If your state does not do emission test and you can live with a constant on check engine light, you should be able to drive the car just fine.

[mcdavidhome]

Yeah, that seems to be pointing to the catalytic converter. I just think the catalytic converter in the Grand Prix is not that long lasting. For the fours years I owned my 2005 Grand Prix base model, I had two catalytic converter broken. At least a bad catalytic converter, unless it is clogged, is not going to influence how the car drives. If your state does not do emission test and you can live with a constant on check engine light, you should be able to drive the car just fine.

Yes my state does not do emission test and I could easily cut my CAT off but I'm afraid that without the cat I would get bad gas mileage due to the O2 sensor readings being off. I have heard some people say you can do the non spark fouler trick to get the o2 sensors to show good readings to fool the Computer but I don't know how reliable that is..