Def problem

Haven’t run into it myself, but code reads as Improper Diesel Exhaust Fluid (DEF) Quality. If you are going to try to fix this yourself, you’ll need to remove your existing DEF and replace it with new, make sure there isn’t anything floating in the bottom of the tank. Replace the DEF filter (mounted in the bottoms of the pump).
The tricky part is meeting the verification process, It needs a regen. if you could still run down the Hwy, you would just need to drive till it regens. Since your locked down to the 5mph, I can’t think of a way except to hook it to a PC.
On a side note in my experience, Detroit’s with DEF (in a Freightliner anyway), if it shows the def tank level with any green bars and a good fuel level, but the orange low level light starts flashing. Do not fill your tanks, def or fuel, that is the trigger for the Government mandate 5mph limp mode. It tells them your in civilization and “safe”. Unless you meet verification of repairs it won’t unlock. In your case you need a regen, if it was with the def air system it’s as simple as running the truck for a few minutes, shutting it off and letting it complete a purge cycle.

 

Unhook Speedo sensor so you can drive 55mph. It may be your def sensor harness? Check connection for corosion, and or bare wires.

 

I has that code and nox efficiency low. Ended up needing 2 nox sensors. Def was good. If its a only code you are getting maybe you can change def fluid and filter. But most likely you need to get to the dealer.

 

My 2015 dd15 is in shop for same as Me Myself and I and your code. Fingers crossed on Nox sensors because the rabbit hole leads to box 1 failure if not.