Def

My truck will do this occasionally, between about 3/4-1/2 tank of fuel then suddenly drops, atleast its manageable if you know what it is.

 

If it derates unplug the speedsensor from the transmission. Your speedo won't work but you will get where you're going.

 

2012 it should be manual, but if it's an auto unplugging the speedsensor will keep the trans from shifting.

 

Ive driven auto shift Volvos with the speed sensor unplugged... It will still shift, but it wont build any boost... So its gutless.

 

Try disconnecting the batteries for a few minutes. I had a similar issue and this worked to reset the system and it let me travel a few hundred miles before it did it again.

Odd thing was it ultimately appears to have been the wire going to the 7th injector (doser) was getting too hot. I noticed the wire lost it's insulation and had dropped down from its normal position along the valve cover and was laying on the side of head.
Once I reinsulated and rerouted it to a cooler location the problem has not reoccurred.

However, I don't recall that my DEF gauge was acting up.

 

I suppose it's a YMMV situation. Had a Mack with the M shift and it wouldn't go higher than 3rd with the sensor removed.

 

Do a parked regen. Should take care of it.