Question

2004 Chevrolet Silverado 4.3L V6 with engine code P0420. Replace both cats and all 4 sensors. Cleared the computer and it came back. Any thoughts? I also recently did a full tune-up plugs, wires, distributor cap, rotor, ignition coil, crankshaft sensor, mass air flow sensor, and air filter. Any thoughts

Answer 1

P0420 Catalyst below threshold Bank 1

Is there any exhaust leak prior to the catalytic converotor,that is capable of leaning the catalytic convertor,Some catalytic convertor needs at least 50 miles of the gas mating surface of the catalytic  to cure, this may be just that . clear the code which will reset the fuel trim and test drive for the code condition

Answer 2

P0420 Chevrolet Code - Catalyst System Low Efficiency Bank 1
Read more: https://www.autocodes.com/p0420_chevrolet.html