Quote Originally Posted by mcdavidhome View Post
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.