Flashing DEF Light and Derate Clears Itself With New DEF

I got the warning that I was going to get a derate in 100 miles, the DEF red light started flashing and the DPF filter gauge swung from almost clean to full. I didn't know that shutting off the truck would cause the derate immediately on the next start. I have had trouble with bad DEF before so I drained the tank and put in 20 gallons of fresh, it had been awhile since DEF filter was changed so I changed it too. My OTR tool showed no active codes except SPN 1569 FMI 31 and SPN 5246 FMI 0. I tried doing resets with the OTR tool but problem would not go away. I left the truck idling and went to sleep. 4 hours later all warnings were gone! The red light was no longer flashing and the DPF gauge had swung back to clean. Thanking God I did not spend thousands on a tow. I'm told it is recommeded to fill the DEF tank with new DEF but in this case 20 gallons did it, it's a 30 gallon tank. Your mileage may vary. I'm posting this in case its helpful for somebody else. 2017 Peterbilt 389 Cummins ISX15.

 

I worried that it could come back but so far so good. To me its kind of crooked that with all the live data and codes there isn't a way to know what the hell is happening. Seems like in the middle of no where with the "stop engine now!" light flashing and the bell ringing you have to guess if its the fluid, the quality sensor, the level sensor, a nox sensor or what have you.