I know that this may seem like the issue that was addressed by a flash recently, but I don't think it is. That issue was when filling with the engine running. In my case, I filled it with the engine off:

About a week ago, my truck was down to 400-ish miles to empty on the DEF indicator. I dumped in a 2.5 gallon container, but it only reset to 799 MTE, and resumed the countdown from there. Since I was only going about 500 miles round trip, I didn't worry about it at the time... but just before starting the return trip, I realized that it has dwindled all the way back down to 300 MTE. Of course, this happened because I was towing... but I was forced to buy some DEF at a truck stop so I could be sure it wouldn't enter "reduced power" mode while I was hauling my fiver back up I-35. Once I dumped in this $20 bottle of DEF, it read "OK" and I went on my way.

This led me to believe that the DEF level sensor can only read "over half" or "under half" and uses an algorithm based on usage to calculate MTE. That would explain the "OK / Under half / 800 MTE" readings. It would also explain why it was counting down even though I had close to 3 gallons in the tank. I must not have gotten it quite back to the half mark, so it continued the countdown. BUT... if that's the case, then why did it reset to 799? It MUST be able to read some granularity between half and empty.

I can't explain this behavior, and I don't think it falls under the TSB fix from a couple months ago. I remember reading that someone suggested always filling to half full or more... and while that seems like it would have resolved my issue, why is it necessary? The meter can obviously tell the difference between 1/10 full and 1/2 full, since it reset my countdown to 799.

Maybe this is just a reposting of another thread that I missed, but I don't think I've seen this particular symptom mentioned before. Thoughts?

 

I just had the same thing happen to me. I have 3800 miles on the truck and had 300 miles left on my screen. I put 2 1/2 gallons in because def is suppose to have a shelf life of 1 year and I do not put that many miles on in a year to use a whole tank. I wonder what happens if your def gets outdated. How will the truck respond if you are running outdated def. I know I have over 2 1/2 gallons and I still have the count down going on.

Ken