I had this happen on our 2014 Allegro Bus 37AP as well this summer. I had a half tank at the time it went to E. I added 5 gal, and it went back up to full. It now just stays between full & 3/4.
What I find out is our MH is only looking to see if there is DEF in the tank. It isn't checking quality, which is the issue that is causing people to replace the DEF head. I suspect you just have a stuck DEF level float/gauge in the tank. (Google that and you'll see how to clean it. And to minimize it reoccurring, you should keep you tank 3/4 - full.)
I spoke to Gary Harris, Tiffin chassis manager, who wasn't quite sure what the issue:
"I suppose it is possible your sensor is beginning to fail, but this is not the typical behavior we see with failing sensors.* *More commonly the level sensor would fail as a short circuit and blow the aftertreatment fuse.* When this fuse blows multiple fault codes are logged and the problem only goes away after the sensor and fuse are replaced.*
*I don’t have an explanation of why this would happen.** I would just recommend keeping an eye on it for a few days and if it happens again you may need to have it checked."
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