Check your 02 sensors take 2 min if you know how to use a scanner make sure they are switching correctly, then check your converters with back pressure probe or heat gun.

Any exhaust leaks before your 02 sensor will case same issue. Check these obvious problems first. Takes a few minutes.

The temperature going into your converters should be colder going in and hotter comings out.

A thermo camera will show restrictions as well. Make sure you have no other codes for the ECM as well. If so, what are they? To have p0420 and P0430 MIGHT indicate an exhaust leak. Any leak before the O2‘s will cause these codes. Keep that in mind. Even if you replace them with new converter assembly, you cannot have any exhaust leaks before you 02 sensor. And never go cheep with GM O2 sensors, you’ll chase your tail.