Thread: Need help with 0420 and 0128 code!!

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

does your engine warm up like normal? if it doesn't then the thermostat is stuck open and that could cause a P0128 code. i've never heard of that causing an O2 sensor issue though

I have noticed that my temp gauge is running 170-175 after driving for 30 minutes it’s not even close to the halfway mark.

Originally Posted by mcdavidhome

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.

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.

Originally Posted by 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.

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.

Hey guys I have an update on the P0128 and P0420 problem I was having. As it turns out like you guys said the thermostat was stuck open thus causing the engine to not reach normal operating temperature which actually caused the P0420 code.

The way I found out was by looking at the live data from my 02 sensors. The pre cat o2 readings and the post cat readings were very similar with the bad thermostat but after I replaced the thermostat the post cat o2 sensor reading changed dramatically. The wave

form was almost flat which would indicate the Cat was doing its job by reducing emissions. I am honestly shocked at how many problems you can get from your engine not reaching the correct operating temperature. I really appreciate you guys helping me solve

this problem.

P.S. I tried to upload a screenshot of my live data so you guys can see for yourselves but was unable to because I exceeded the size limit by 25kb. I will see if I can shrink the image and try to post it later. Once again thanks for your help!!

Originally Posted by mcdavidhome

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

UPDATE!!!! Hey guys I have an update on the P0128 and P0420 problem I was having. As it turns out like you guys said the thermostat was stuck open thus causing the engine to not reach normal operating temperature which actually caused the P0420 code.

The way I found out was by looking at the live data from my 02 sensors. The pre cat o2 readings and the post cat readings were very similar with the bad thermostat but after I replaced the thermostat the post cat o2 sensor reading changed dramatically. The wave form was

almost flat which would indicate the Cat was doing its job by reducing emissions. I am honestly shocked at how many problems you can get from your engine not reaching the correct operating temperature. I really appreciate you guys helping me solve this problem.

Originally Posted by mcdavidhome

Hey guys I have an update on the P0128 and P0420 problem I was having. As it turns out like you guys said the thermostat was stuck open thus causing the engine to not reach normal operating temperature which actually caused the P0420 code.

The way I found out was by looking at the live data from my 02 sensors. The pre cat o2 readings and the post cat readings were very similar with the bad thermostat but after I replaced the thermostat the post cat o2 sensor reading changed dramatically. The wave

form was almost flat which would indicate the Cat was doing its job by reducing emissions. I am honestly shocked at how many problems you can get from your engine not reaching the correct operating temperature. I really appreciate you guys helping me solve

this problem.

P.S. I tried to upload a screenshot of my live data so you guys can see for yourselves but was unable to because I exceeded the size limit by 25kb. I will see if I can shrink the image and try to post it later. Once again thanks for your help!!

Dropbox - Bad thermostat..jpg - Simplify your life

Dropbox - New thermostat..jpg - Simplify your life

Thanks!

Originally Posted by Coldwaves

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..