99 1500 gmc suburban code P0430

i replaced al o2 sensors on my suburban. had engine turned off and came back on 2 days later with the same code.
had friend look at it and at least mechanically checked out ok.
he checked the cats and the temps on them were consistent.
checked pipes going to cats they were consistent
checked downside pipes they were conistent and cooler than frt side
cats were not noisy like they were broke on the inside.
only thing i hvnt done is check voltage on the sensors themselves

any suggestions?

k1500 5.7 350
thanks
jeff

You cannot use temperature to check converters for a low efficiency code. You will need a scan tool to monitor the the upstream and downstream voltage of the o2 sensors. But 98.9% of the time it's a bad converter. Also get a GOOD quality one if you replace.