Very Fast DEF Usage
I now have 6,000 miles on my 2017 Duramax. Twice now I have had the 30% DEF caution come on and at the same time have had DEF mileage remaining warnings showing only 130 miles and 160 miles. On both occasions, the DEF level went from 30% to reading "Low" after about 50 miles, along with the caution that says something like "DEF Low - engine will soon be speed limited". In my 2015 Duramax, 30% DEF corresponded to approx 1000 miles remaining, now it seems to correspond to about 150 miles remaining. Also, assuming that the DEF tank was full when I got the truck and again full after a service where they said they topped up the DEF tank, both times I got the 30% DEF light after about 1000 miles of towing a 7500 lb travel trailer. I have now used 4 tanks of DEF in 6,000 miles (most of those miles towing the 7500lb travel trailer)
Is this normal? Anyone else seeing this behavior? It sure seems like my truck is burning through DEF at a rapid pace.
P.S. The first time this happened, I had to find DEF in the middle of nowhere, Montana, and filled up with some unknown brand of DEF. Shortly after I got a CEL The code was P0821. This happened again 3000 miles later - same code P0821. The fix, applied after the second incident, according to Chevy dealer was to reprogram the TCM and ECM. No CELs since that fix in around 1000 miles of towing.