Hopefully someone can help. I have a 2002 silverado . I've never had a p0420 code or issue before and iv had the truck for 4 years. It always passes smog. Except this year I got the code. I installed a new cat and cleared the code drove it around and the service light came on again. I checked the code and it's the p0420 code again. It says catalytic emissions below threshold. I dont know what other problem it can be. Anyone have any clues. Thank u.

 

did you buy a cheap cat? bought the complete ypipe from reputable name?
the other faults for this code are bad 02 sensors or leaking exhaust.

 

bad O2 sensor it would be behind the cat. could also be from a cat flowing too much air (high flow cat or exhaust leak)

 

if you had a lazy precat o2 the fault will set. the cat monitor compares from and rear 02 sensor cross counts to determine the fault.

 

I didn't buy the cat from a dealer. I payed 260 for the cat. I cut the old one off and welded the other one in.

 

I do have some exhaust leaks but there before the cats

 

hey tech i love you, but not in a bad way lol. i have experience with older chevy motors (pre 96) but not so much with these new ones. im learning more and more from you, thank you. for some reason i cant keep the concept of an O2 sensor being "lazy" in my mind they either work or they dont.

 

where is the leak, before or after the first O2?

 

how the monitor works...comparing cross counts/switching of the o2 sensors...the front should switch at a much faster rate than the rear...when they switch at the same rate...computer thinks the cat isn't working.
fix the exhaust leaks. clear the code. if the fault resets....and the o2 sensors are working properly...its the cat. Cheap cats have low cerium content....the oxygen storing element in the cat. I have seen cheap new weld in cats set the code. putting in a quality part was the fix.
so, I would do this. clear codes after each step. if the code does not reset...no need to proceed with the next step.
-fix leaks
-make sure 02 sensors are good
-replace cat

 

The exhaust leaks are before the cats.