Your code indicates a problem with the forward switch located on the control handle. it may be out of adjustment or not working. Early NRAD has spade terminal connections. later NRAD truck series had sealed switches in this location. The handle being a moving component may cause looseness in the connections or broken wires over time. Also checking the potentiometer calibration may help as you may get this code if the truck thinks you are out of neutral position without the forward switch activation.
Specifically the logic is the main Interface Board (MIB) sees a high pot signal (2.6 volts or higher and the forward switch is in the NO position, but the handle is not in forward. Also ensure the control handle is returning properly. The control handle springs may be worn. Finally you may have a voltage drop. Wire #60 pin J1-13 should be at about battery volts when the handle is in forward(closed switch position). A short or loose or corroded pins at the MIB J connector should be checked.

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