2002 Yukon p0420 and p0430 codes when smogged

Question

I have a gmc 2002 yukon with codes p0420 and p0430. everything passed on the smog except for the OBD system check. The worst reading was the NO(ppm), the max is 472 and it read 410. Everything else was well below the max. What might be the cause of this issue?

Answer 1

P0420 Catalyst efficiency below threshold (bank 1)

P0430 Catalyst efficiency below threshold (bank2)

You have a three way catalytic convertor, a reducer and a oxidizer , The oxidizer part of the catlytic oxidizes HC (hydrocarbon) and CO (carbon monoxide) which converts these gases into harmless water vapor and carbon dioxide. the second  part of the catalytic is not  working,The reduction part reduces Nox into nitrogen , When you code P0420 and P0430 the TWC (3way catalytic)oxygen storage capacity is below acceptable threshold, The solution is a replce catalytic convertor