My '99 Suburban 5.7 L with 135k miles keeps giving me a PO420 error code. I have changed all the O2 sensors; the light was reset and stayed out about 1000 miles and then came back on. I swapped sides for the sensors to rule out a bad sensor and it came back on after about 500 miles. I find it hard to believ that the catalytic converter is bad and only on the one side.

Anybody have any other ideas?

 

it's prollyur converters. i've got that code several times and the 02 sensors were never the problem it was always the converters.

 

well i know a way to fix it but i wouldnt say that it is legal but it is most likely your cats the only other thing it could be is a leak in the exhaust after the first o2 sensor and before the second.