2002 Silverado 5.3L (LT) Check Engine Light P0440 and P0420

[Gito]

2002 Silverado with 415k miles.
California Emissions

Have a check engine light and getting codes P0420 and P0440 (Bank 1).

Muffler shop says its probably the catalytic converter on the drivers side but also mentioned that it could be something else.
They will replace it if I ask them to.

Truck has the original catalytic converter on the drivers side.
I don't want to replace the catalytic converter to later find out that its not the problem.

What else could be triggering these codes. I've actually been driving with a check engine light for at least a year.

I replaced the upstream and down stream sensors on the passenger side a few years ago.
Thinking about replacing all of the 02 sensors and then go from there.

Any advice is greatly appreciated

 

[HoosierJeeper]

Probably worth trying the bank 1 O2 sensors if they're easy to do.

 

[AutoMechanic]

I’d go with doing the Bank 1 oxygen sensor first and see if that fixes it that would definitely be your cheapest option.

 

[clinebarger]

P0440 is a Evap leak code......I replace a lot of Vent Valves for this DTC, But needs to be smoked to verify. (With the Vent valve commanded closed).

Rarely are P0420/P0430 Catalyst Efficiency DTC's caused by faulty O2 sensors.

 

[Gito]

I might end up replacing the catalytic converter.

Air Resources Board (California Emissions) requires me to use only an approved catalytic converter.

There is number printed on a sticker (2GMXA05.3187), which is located on the air filter housing cover, that is listed on the Air Resources Board website. The website provides a list of approved catalytic converters. The Magnaflow #459006 is on that list.

I checked catalytic converter prices and they are all around $500.

Going to replace the 02 sensors regardless. They have never been replaced and we have around 415k miles on the engine.