Running out of DEF in a Duramax


Staff member
I've found in both my 2018 and 2021 Duramax, that the DEF gauge in the DIC lies about the DEF level. The DEF tank hold 7.0 gallons according to the manual, the same as my 2018. We have been getting about 325 MPG DEF while pulling, so estimating the amount left is pretty simple.

Normally the DIC will pop up a message saying 1000 miles until DEF tank empty, but the gauge still shows about 3/4 tank, which is about 1,700 miles DEF still in the tank. The DIC will show messages reminding of your depleting level of DEF, until you "run out".

Today, I got a message saying "DEF low refill now, speed will be reduced soon" about 50 miles short of my destination. I got a bit nervous, but the numbers didn't add up. My math said there should be about 2 gallons left in the tank.

I got about 10 miles from my destination and the when this happened, the DIC displayed the following messages and locks the DIC so you can't see anything else.



I got to my destination and refilled the DEF tank at a SAPP Brothers truck stop. Guess what? I wasn't out of DEF, instead had 2.5 gallons left, enough for about 800 miles.

Why the games GM? Why not just have an honest DEF gauge?
Is that dealer programable? can it be calibrated? I'd take it in and ask, seems like a 1/2 gallon would be the place to be alerted.
Is that dealer programable? can it be calibrated?
I don't know, but should ask. However, the fake alerts are widely complained about on internet forums, so I'm guessing the EPA demanded the gauge and messages be programmed as they are. The guy parked next to me has a new Ford diesel and he said his is the same way, dire warnings when you have 500+ miles of DEF left.