Code 5 is a start switch or brake switch code. This truck has a Gen 2 Sem traction controller in it. SX panel. Does this truck have contactor hydraulics or a SP sem controller for the hydraulic side? If it has contactor hdraulics, Check the diode suppressor on the contactor. it mounts between the two screws on the coil. You said you replace a drive module? Not sure if you mean a sem controller? Controllers have to be programed and set up for the right truck & voltage. You can use a GE LX Handset or lap-top on this panel. Handset only plugs into controller in the rear. Lap-top can plug into controller or behind your dash display. Let me know if you contactor or sem hydraulics? What driver module did you replace? I can send you the right schmatics for this unit? Hoe this helps alittle. ETS1

The code 5 is an 'sro' code that says that the start switch (the one that clicks as the accel pedal is depressed) fails to close.

I assume that both problems happen all at once. If so, the likely cause is the keyswitch or related wiring. Both circuits share a common positive source.

The pos. source starts at the fuse, then keyswtch, then goes on to feed all the switches. It sounds to me that as the load increases the voltage drops.