Code P0420 how do I fix?

DTC P0420 is indicating a problem with catalytic converter which is common when they where out here is a video that sows how to fix it.

https://youtu.be/CORe9JyrexI

Please let us know if you need anything else to get the problem fixed.

P0420 - Catalyst System Efficiency Below Threshold (Bank 1)
P0430 - Catalyst System Efficiency Below Threshold (Bank 2)

To be detected, these faults required properly-working oxygen sensors. These indicate there's no change taking place in the composition of the exhaust gas as it passes through the catalytic converters. This article will do a better job of explaining how the system works:

https://www.2carpros.com/articles/how-emission-control-systems-work

You may find more information here too:

https://www.2carpros.com/articles/smog-test-inspection-information

https://youtu.be/kNXh1Lphzr4

https://youtu.be/CORe9JyrexI

Hello, I'm Danny.

Just to add here. I'm curious to know with having 275,000 miles on the truck. did you use the old exhaust system with the old catalytic converters? Those fault codes are normally for the catalytic converters. The o2 sensors just act as a monitor. Here are tutorials showing symptoms, and how to test a catalytic converter:

https://www.2carpros.com/articles/bad-catalytic-converter-symptoms

https://www.2carpros.com/articles/how-to-test-a-catalytic-converter

Let us know what you find out and get back to us. Hope this helps and thanks again for using 2CarPros.

Dandy answer.

If it helps, the Engine Computer is watching the switching rates of the O2 sensors. The front ones switch from "rich" to "lean" about two times per second. If the catalytic converter is working properly, the rear O2 sensor will switch perhaps as little as once per one or two minutes. As the converter loses its efficiency, the rear O2 switching rate becomes faster and faster, approaching the same switching rate as the front sensors. At some point the software decides, based on the increased switching rate, the converter is not as efficient at cleaning up the exhaust gas as it needs to be, then it sets one of the codes you listed. That catalyst monitor has no idea of the engine size. If you're using the original computer with the original O2 sensors, the only thing that will be different is exhaust gas volume, but there's nothing measuring that. You can't even say that's measured by the mass air flow sensor because exhaust gas is very hot and has expanded a lot.