P0420 Code Set need troubleshooting advise

[steveO]

The Envoy recently set the P0420 after a long drive.

Engine runs fine and smooth

What I checked:
Exhaust:
The exhaust is not leaky I check it from end to end, thought the flange gasket to the manifold might be leaky so I replaced it for good measure nothing else show signs of leaking.

Engine Temp:
I measured this using a scanner and it runs a bit cooler than it used to, when running it varies from a high of 197 to a low of 194.
The happy temp seams to be 195.8. From what I recall this is on the low side but not low enough to throw the low temp code... Maybe it's low enough to cause the P0420?

Pre and post o2 sensors:
I took a look at the readings from the scanner they bounce around a but post seems to settle down to a more linear ware form most of the time. At times when the temp is cooler they appear to be similar in wave shape.

So at this point I'm thinking I should attack the engine temp first and see if it elevates the temp by replacing the T sat and see if the code goes away.

Any additional suggestions are appreciated.

 

[The_Roadie]

The post-cat sensor should not move at all. I'd guess you have a cracked cat that mostly seals up when hot.

 

[jake 03]

The diagnostic for p0420 (bank 1 catalyst efficiency low) is very straight toward. The cat is not doing its job and needs replaced.

 

[C-ya]

The diagnostic for p0420 (bank 1 catalyst efficiency low) is very straight toward. The cat is not doing its job and needs replaced.

Or the post-cat O2 sensor could be hosed and not seeing a change that indicates the cat is doing it's job. For the cost of an O2 sensor vs. a new cat + installation, I know which one I would try first.

 

[steveO]

The Envoy recently set the P0420 after a long drive.

Engine runs fine and smooth

What I checked:
Exhaust:
The exhaust is not leaky I check it from end to end, thought the flange gasket to the manifold might be leaky so I replaced it for good measure nothing else show signs of leaking.

Engine Temp:
I measured this using a scanner and it runs a bit cooler than it used to, when running it varies from a high of 197 to a low of 194.
The happy temp seams to be 195.8. From what I recall this is on the low side but not low enough to throw the low temp code... Maybe it's low enough to cause the P0420?

Pre and post o2 sensors:
I took a look at the readings from the scanner they bounce around a but post seems to settle down to a more linear ware form most of the time. At times when the temp is cooler they appear to be similar in wave shape.

So at this point I'm thinking I should attack the engine temp first and see if it elevates the temp by replacing the T sat and see if the code goes away.

Any additional suggestions are appreciated.

I don't do as many long drives with it as I used to so that may be the reason it hasn't set the code.
The current cat is a 5 years old direct fit replacement. So forget the T stat and just replace the cat.

Thanks

 

[budwich]

you likely have another issue causing your "cat to get eaten"... rich running due to temperature issue or small coolant leak, etc. Anyways, its just money and time.