Hi Busch,

I assume you started with the basics ie. Battery status, Shorts to chassis, Brush/comm condition, Motor cables, Static strap in place, Horn not shorted, main fuses checked, Heat sink insulation doing its job and contactors checked. I'm basing this on a hard fault. Intermittent faults are tough.
Based on the Code FG, I assume your working on an Order Picker? Not sure if it's the new format on this website but I can't see any truck details.
My advice is to start with the above before jumping to any of the diagnosis in the manual.
Hope this helps,
Jay