Def

Maybe so, but all I know is I once had a similar issue and it did fix it.

 

Not sure if this will help but with our kenworth t800 and 900's we constantly have scr/def fault lights come on with the red light on the def gauge, derate in 30 minutes, we just disconnect the return line on top of the def tank blow air through the fitting going into the tank to prime the pump, wait till you see fluid coming out of the line you just disconnected, reconnect the line and within a minute all fault lights go out and it runs like a champ. It'll be good for a few days or longer. Company's to cheap to buy new def pumps. Whatever it takes to get me back on the road.

 

I drive a 2019 Volvo. I was 90 miles from the nearest Volvo dealer when I got the same fault. Company wanted me to take it to the Volvo dealer. I didn't want to get an hour down the road and have it derate on me. I talked to the guy at the Volvo dealer and he told me that as long as I do not come to a complete stop, it will not derate to 5mph on me. Sure enough, I made it ( there was only 1 stop sign to deal with and I just rolled through it, I got lucky).
Anyway, dealer fixes it and tells me it was actually having a fuel filter problem and they replaced them and it's fixed now. I was dubious, but not my truck and not my mechanic bill. Whatever they did, it did fix the problem and it hasn't happened since.

2019 Volvo VNL 760 with a DD13 engine.