There is no cooling for the DEF, so when the tank runs low after long running durations the tank temperature can increase. Also, the DEF constantly flows through the dosing module on the exhaust to keep this unit cool. This heat is returned to the DEF tank. Ensure that the Coolant Control Valve (CCV) is not flowing engine coolant through the DEF storage tank. Check the CCV for leaking, check the line connection and check if the coolant lines are warm to the level unit. Check the tank heating valve. Ensure the heating valve is in normal operation, not stuck open. If the heating valve is stuck open, replace the heater valve. If the valve is in normal operation, erase fault code and continue operation. The DEF tank heater control valve may stick intermittently. If the fault returns repeatedly, replace heater valve. On tractors built in Fargo, the DEF heater line with the 'red' band must be connected to the top tube which runs to the front part of the engine head. This may or may not be true for tractors built in other plants. If the DEF tank hoses are installed wrong, hot water can circulate continuously and overheat the DEF tank. If the hoses were connected wrong, the valve can be damaged and may also have to be changed to correct the overheating.
Check ballasts under back fender. There are 2 bolts holding the brake light cover. Remove to gain access to the ballasts. Check voltage on ballasts. Swap ballast wires from known working light to the non-working light. If the light works, replace ballast. If the light does not work, you may have a bad bulb or light assembly.