66 BATTERY LOW
When the battery charge level reaches 20% or less or (10% on the dash display), the current level for the drive motors is reduced to 50% of the
programmed level, the travel speed is reduced.
Traction (M)
* Charge the battery.
* Check the condition of the battery and perform a battery load test to determine if the battery is bad. If necessary, replace the battery.
* If the alarm continues, replace the controller.

Master controller, first replace the 500 amp fuse then if not it tap controller with screwdriver on front and see if it moves if so replace it

The code is a dead battery try charging the battery it is possible that the battery is weak and after charge the cell do not hold to what a good battery would see this could be why the code is coming on if so try to cheat the controller by charging the battery but do not let it go to the end of the cycle shut off the charger and plug back into the truck and see if it clears. if so the battery is weak. You could try clearing with the zappi hand set.