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
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.