Have a 2003 Carlo 3.8 engine. 57000 miles.
Check engine light on, code P0420.
Replaced down stream 02 sensor for first try on repair.
Cleared code check engine light off, ran vehicle for 4 days to
reprogram, but check engine light came back on,
code P0420 came back.
Cat. shows good, would the next thing to try be the upstream 02 sensor?
Thanks! for any help.

 

90% of the time when this code comes up...its your catalytic converter. but could be the o2 sensor behind the cat, or a exhaust leak. but im assuming your car doesnt have a leak cuz you would be able to hear it.

 

if you were to get an aftermarket car this code would come up, this is because the after cat o2 dont register. do you have an aftermarket cat by any chance?

 

Have replaced downstream O2 sensor after Cat.
Cat. is original and checks out OK. No leak in exhaust.
Could it be the upstream O2 sensor, by the manifold?

 

the 02 sensor wires could be damaged. or the sensor could have just simply killed over. i had my hubby replace my o2 sensors cpl years ago and after he changed them i had more codes than what we started with so i was forced to put one of the old 02 sensors back in and the codes went away. there was something wrong with that sensor. could be your problem. usually the upstream wont have anything to do with that p0420 code tho. and if you ran it for 4 days with no trouble, then it came back...i would look into that sensor again.

 

How is the cat getting checked?

What brand o2 sensor did you replace with?

 

Cat. was checked at a muffler shop.
O2 sensors are Bosch Brand.

 

Some people have reported issues with Bosch sensors, might be worth trying to swap for an ACDelco and returning the bosch?

 

Me personally had issues with the bosh upstream. I went right back to ac delco

 

I can attest to another brand of sensor tripping the code... I took the sensor back and paid another $10 for a Delco.. no more code.