2021 Duramax L5P CEL - P20EE: SCR NOx Catalyst Efficiency Below Threshold Bank 1


Staff member
Aug 20, 2003
Naperville, IL
We were driving down a steep grade using the exhaust brake yesterday and got a CEL. I used my Banks iDash to see that it is a P20EE code, which means, "P20EE: SCR NOx Catalyst Efficiency Below Threshold Bank 1". I did a bit of research and found that there are two NOx sensors, one just after the turbo and one after the SCR catalyst. There has been a rash of the first sensor failing.

I called Dellenbach Motors in Fort Collins and asked if I could make an appointment. They said sure, we have an opening in 10 days. Ouch. I explained we are traveling and weren't planning on being here in 10 days. They said they understand and that I could bring it in and they would look at it as soon as possible. I took it in and GM got me a rental (Chevy Spark :oops:).

I didn't plan on hearing from them for several days. I stopped about 5 minutes away for a coffee and about 30 minutes later I got a call from the dealer saying the truck was done! What, done? There is a TSB, 21-NA-128, that calls for a reflash of the ECM for the P20EE code. Seems there is a software bug that makes the ECM think the sensor isn't working correctly. My guess is their diesel mechanic said he knew with high probability what the cause was and wanted to get me back on the road as soon as possible. Very nice of them.

Hopefully this fixes the problem and we don't see the P20EE again.

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