Got on 04 3v and had to have my cats replaced last year. Had the rears cut out and the fronts replaced (clogged).

Now I am getting code P0420 again a year later. Shop said the cats are under warranty, but just wanting to make sure I solve the root of the problem. Can't keep throwing converters at it.

OBD shows no misfires, Mode 6 shows no misfires, but sometimes you can feel a shudder at idle while sitting in traffic. Its got the Champion plugs with about 80k on them. Replaced coils with engine 80k ago.

I am going to do plugs and boots again with the MC's. Not sure if I should be looking at O2 sensors as well, they will have about 210k on them now. (assuming original owner never replaced them) I have put almost 100k on the truck in the last 4 years.

Any suggestions before just getting cats replaced?

 

The entire concept of the 'closed loop' fuel/air mixture system is based upon voltage readings of 0.45 volts from the O2 sensors. They are known to get 'LAZY' with age/mileage and react to changes in oxygen content more slowly.


If they drift from factory calibration and perfect oxygen content starts reading 0.3 or 0.5 due to accumulation of exhaust residue or contamination from fuel additives - the whole system is screwed. I am a firm believer in replacing them as preventive maintenance every 100,000 miles.


Also - catalyst efficiency monitor tests are based upon rear (Post-Cat) O2s reaction times to PCM's variations in fuel trims, and the 'ratio' of voltage switches between front / rear O2's. It would not be 'far-fetched' to think this could result in P0420 or P0430 codes.


At your 210K (knowing YOU have put 100k) on the truck, I would recommend replacing all four O2's.

 

Thanks, I will be replacing the sensors as soon as the weather improves.