2020 International HV DEF gauge not working, a few days after constant shut-downs.

Three days ago, I was trying to clear snow in our plow truck, when it suddenly started dying on me. Not even above a very high speed, maybe 4-5 mph. Sometimes I'd be able to get a little faster, go ten or fifteen feet, then the engine would shut down, sometimes even while going a bit slower but at a higher RPM (due to driving through snow). It took me forever to limp back near enough to the office to switch to the loader to continue plowing.

So after a bit of research, that looked like a DEF sensor failure. I topped off the DEF completely in case that would help anything, and the gauge didn't move from less than a quarter full, and it would still die. I then spent yesterday morning calling around Navistar dealers to get someone here to replace it as soon as possible. I still haven't got a confirmed appointment, I'm a ways away from anyone and there's more snow due ... but now the truck's running fine. Decided to give it a test run a half hour ago, and everything ran just fine, up and down the runway & taxiways, clearing snow or just driving up to 40mph.

Except the DEF gauge still only shows less than a quarter full, when it's completely full. The only thing I've done is pull the DEF header fuse from the PDM and put it right back in (it was fine).

Sooo ... should I continue to work at getting the whole DEF sensor package/heater valve replaced, per Navistar's notice from a year or so ago? Or if it's just a stuck gauge, can I knock it loose? That gauge makes me mistrust the whole system.