Gem cart won't engange and drive code 15

My gem cart won’t drive but it’s showing code -15 when I turn it on and all batteries have 12+ volts

Start here.

Total voltage at the controller is probably less than 68 volts. Fully charged set of batteries is 78+ volts. Charge each battery with a dumb charger to at least 12.5 volts and try it again. Clean all connections including those on the controller.
'Bright and Tight". Test the voltage at the battery pack connection points for 70 volts or more.

Unplug the 23 pin connector on the controller and spray with deoxit.