Iíve got a 2012 GMC 2500
HD Duramax/Alison with a shade over 25K miles on it. 95% of the miles have been pulling one of two trailers (FB 27 FC and a 17ft Casita). My best guess is that it seems to get close to the advertised 5K miles per DEF take full. I say, best guess since initially, before trips when the tank mileage was in the 3-4K ish range Iíd refill the tank. I never let the DEF level get low enough for the warning message. Then on a trip about as far from a dealer as you can get, the engine/ emission warning light came on and a bit after that, the warning that in 170 miles speed would be reduced (I was 140 miles from the nearest GM dealer). Turned out the emission sensor (donít remember the exact name) had failed and needed to be replaced. No big deal under warranty but a major cost if out of warranty. First question the dealer asked me after they ran the diagnostic tests and confirmed it was indeed a faulty sensor, was did I refill the tank before for low fluid warning message. Said yes, reply Ė thatís your problem. Should not refill until the message for low fluid comes on. When we got back home it was time for servicing so I took it into my local dealer. Told them about my problem and again their first question was had I refilled the DEF tank early. Again the response was donít do that! That was probably the cause of your failure. I was told there was even a service bulletin out telling dealers not to refill the DEF tank before the warning message comes on. Now, I have no idea why refilling early would cause a sensor failure but at this point Iíll wait until the warning comes on. And carry a supply of DEF fluid with me.