VXDIAG VCX NANO GM Monitors Sierra Duramax 2011 NOx Sensors

Vheicle: 2011 Sierra Duramax 142k km



“Poor quality DEF” message out of nowhere.


Attempt 1:

First I tried topping up/diluting existing DEF and going for a 30 min hwy drive.

I ordered a Tech 2 equivalent diagnostic tool online to monitor NOx sensors (VXDIAG VCX Nano GM).

Asked the local dealer how much to re/re the DEF tank…3.5 hours!


Attempt 2:

Got home, took out the tank, drained it (there’s no ball valve, just a plastic flap) and reinstalled tank in an hour.

Put in 1 jug (2.5) gallons new fluid. Originally no codes or CEL, just the DEF light and the “see owners manual now warning” Started up and had P20EE (failed NOx sensor) and a P207F (poor reductant fluid quality) codes which wouldn’t clear with my tuner.

Went for a 25 min drive, def light cleared, both codes cleared with tuner (Edge CTS diesel) all good.

In my case I figured it was the actual fluid because I only drive a couple thousand miles a year in this unit hauling my 5th wheel which isn’t near enough to cycle through a tank of DEF annually. I’d definitely recommend actually draining/changing the fluid before replacing sensors or injector.

So quick, easy and cheap.