I'd swap the downstream sensor. P0420 is nearly always a sensor problem. That said, catalyst does go bad. But so do O2 sensors, and those are cheaper!

 

A couple of GM bulletins. P0420 is fairly accurate. An automaker would truly hate have false failures & get scrutinized by the EPA.
I like to delete the fault code once, if it returns then it is probably real.

 

A couple of GM bulletins. P0420 is fairly accurate. An automaker would truly hate have false failures & get scrutinized by the EPA. I like to delete the fault code once, if it returns then it is probably real. . . . .

This vehicle belongs to a nephew that works at the GM assembly plant in Kansas City, KS. When we meet he and I have "spirited discussions" about GM engines and poor reliability. I am truly amazed that a GM engine would last this long without catastrophic failure.

I will forward the documents to him and discuss. I will tell him to erase the code, drive the car and see if it returns. The State of Kansas does not have annual safety and emission inspections so he can continue to drive into the future with no registration consequences. Fuel consumption may increase slightly but I am thinking he will NOT want to invest the money on a remedy with that high miles on the engine.

As always I appreciate the insight from ImperialCrown.

 

Replacing an oxygen sensor is not that expensive, and it can prevent actually poisoning the catalyst.

 

. . . .Replacing an oxygen sensor is not that expensive, and it can prevent actually poisoning the catalyst. . . . .

I understand one solution that is very less expensive than replacing the catalytic converter is to "fire the parts cannon" and replace the oxygen sensor. You may get lucky and resolve the issue. But my question that I asked has not been answered. Many YouTube videos on downstream oxygen sensors indicate that downstream trace is valid for a healthy catalytic converter. One way to know for cetainty is to view the upstream and downstream oxygen sensor traces on a known, good catalytic converter system.

In this example of scan tool traces the downstream oxygen sensor does not cross the 450 millivolt line. This is the line than separates lean from rich. On a Chrysler vehicle code P0139, slow downstream oxygen sensor response, would be set if this occurred. On a Chrysler logic that monitors the catalytic converter through the downstream oxygen sensor expects some signal indication that it is going from lean to rich. The sample trace shows it staying in the rich condition; 600 - 800 millivolt range.

Maybe GM logic that monitors catalytic converter operation is not that robust and does not track slop response in an oxygen sensor??? Or maybe GM sloppy engineering logic and a misleading code P0420 is set when in fact the issue is with the downstream sensor???

 

The O2 sensors are used to test the cat. The O2 sensors are supposed to pass their tests before the PCM moves on to test the cat.
If the O2 sensors are getting to be on the 'cusp' of passing or failing their test, then I could possibly see a 'false' P0420 occurring.

 

The other thing that can cause Cat codes or O2 sensor codes is oxidized terminals on the O2 sensors and the connectors, I had an intermittent O2 code on my old 98 Jeep (high voltage for the downstream sensor) for several years, it would come and go, after cleaning the terminals with contact cleaner it's been gone for a long time.
Changing the sensor could be needed given the mileage on the vehicle, but getting them out, can be a real trial. Need heat and a good wrench/specialty socket and PATIENCE. I had to change the upstream one some years back and had to warm up the engine to get the sensor to budge, and also, you might need a thread chaser to clean up the threads in the bung to be able to thread in the new sensor. Use OEM sensors if you can get them!

 

I've had two of the same code and in both cases replacing the O2 sensors fixed it, so I'm inclined to do that. I didn't test them. The right socket did help. It was pretty easy in my case - I think it was PT and 300M. I think the Neon maybe as well? for a third. The sensors don't last forever.

 

I had the P0420 code pop up recently on my Ram. Reset it and haven't seen it since.

It came up before long ago. I changed the spark plugs at that time and haven't seen it until recently.