What's the difference between fuel doser and def doser?

well doser injector is one thing, it sprays a bit of fuel into the exhaust as it leaves the turbo to help burn in the filters. The def doser is probably spraying the def fluid. Idk for sure though, never owned a def truck. But I think that’s what it is. 2 separate things. One sprays diesel, one sprays def

 

No. The doser is at the entrance to the scr not DPF. That's what we call the 7th injector.
7th injector injects diesel fuel to heat up the DPF.
Doser injects ammonia into the scr from the def tank.
Any and all after treatment issues will cause a check engine and set you into derate. EPA norms my friend.

 

They are two different parts.
One injects Diesel fuel after the turbo, before the DOC (Diesel Oxidation Catalyst) to raise temperatures to do a regeneration.
The other injects Diesel Exhaust Fluid after the DPF and before the SCR (Selective Catalytic Reduction) to reduce the NOx in the exhaust.
What codes did you get on the dash?

 

Check those dash codes .