sleepywheel,
the "F" indicator is the Failure Protection Circuit warning code
if any condition is triggered that could cause the operator to loose control of the truck speed, or a condition in one of the drive circuits the logic deactivates the line contactor which shuts the truck down.

you can try what mrfixit says and check the connections at the logics but also direct your attention to the accelerator, it has led pickups inside it and if any of those gets dirty and causes any of the segments of the acceleration circuit to not function it can also trigger this symptom.