0420 code

Check engine light a week ago..
This code. Replaced bank 1 upstream sensor..
Today, same code. OBD2 scan shows only difference in bank 1 and 2 in oxygen sensors is output voltage. 0.74 in bank 2 ...0.05 in bank 1 ..output voltage. Is this a bad wire in harness or maybe bad connector...no clue. Any thoughts??

 

P0420 is bank 1 catalyst efficiency, you need to verify the O2 sensors are operating properly and there's no exhaust leaks, if everything's good there you most likely have a failed catalytic converter.

 

Most importantly, avoid Bosch and go with Denso.

 

If you have a V6, swap your two upstream sensors. If the code switches from 0420 to 0430, its your O2 sensor. If it stays the same, it could be your cat on the way out or wire harness that's giving you trouble. I had a sneaky 0420 and intermittent 0430, took my 02 sensors out and cleaned them with brakleen, dried them out. I also picked up some spark plug non foulers. You'll need 2 sets of 2 if you have a v6. Drill out the hole on one of the non-foulers and screw it into the other. The hole is to fit the O2 head into. Screw everything back into the bank and reconnect. After a few cycles, my issue went away and I haven't seen it since. Bear in mind that this will more than likely fail a visual inspection so you bear that risk entirely. Also to note, I have JBLT headers and don't have cats so I run a rear o2 simulator from URDusa. I assume that my O2 sensors were subjected to higher carbon fowling due to nothing being filtered through the Cats so they failed earlier than expected. Which is why cleaning and the non-foulers fixed my problem.

Bottom-line: Swap your two O2 sensors, see if the problem jumps banks.

-J