Possible Causes and Corrective Actions
* Master Control ON/OFF switch activated
* Low voltage to power amplifier (especially while lifting or driving) due to over-current or short-circuit.
* Try a known-good battery
* PTC R1 missing, faulty, or tripped (very high resistance, due to excessive current while main contactor is open; should automatically recover).
* Carefully check to see if R1 is hot (tripped). If so, unplug battery and allow it to cool. Locate low impedance path that tripped R1 and correct.
* Wire #24 at 0V due to J5 not connected, blown fuse, or bad wiring. * Check for 24 V at J5-16
* Short or open circuit in CAN wiring, or electrical noise in CAN wiring due to bad connection or outside interference, both at power ON.
* Check CAN wiring
* Try operating truck in different environment
* Bad power amplifier

I would check the battery state of charge and make sure it is good and not dropping voltage......