Code 5 also applies to the newer AC drive systems, some if not all of the original SRO codes from GE apply to these systems. Good ole GE keeps things good for us.

check your brake switch or the accelerator input (switch if it has one)
anything in the start circuit can trigger this code from a bad switch to bad wiring, defective potentiometer etc

assuming this truck has a GE SX controller, the circuit is on P3 circuit on the controller to the accelerator start switch according to GE. You can find the GE manual for this controller on fsip.biz (flight systems)

Code 5 is a standard GE SRO code for accelerator start switch not closing. This should be the problem.