I had a code to replace my downstream O2 sensors so I did. The code went away. Drove 20 miles and got a P0420 code. Ive downloaded the service manual and the first thing it says about that code is check prior service records and see if downstream sensors were replaced and if so change tge upstream as well before doing anything else. Anyone evere hear of this or run into it before?

 

Common codes.

Check the electrical connection on the O2. Also check for any cracks in the exhaust manifolds or broken/loose exhaust bolts

 

Ive checked the exhaust and also checked for vacuum leaks. I do have a scanner and all the sensors read from .1-.9 volts

 

got those codes from removing cats. just did non foulers and code is gone and the sensors are in a readiness state to pass emissions. 5 or 6 years code free.