Error code 51 is Capacitor C1 does not charge on start up.

Indication of fault. Line contactor L does not close

Possible Cause
Capacitor bank has a problem(the 3 big blue capacitors on the power block)
Check the capacitor bank for a bad capacitor(short circuit or low resistance). the capacitor bank is charged through a resistor during start up (the precharge resistor is mounted across the line contactor, check the resistor for open circuit). If the capacitor does not charge correctly, the resistor becomes hot. The control card checks the temperature of the resistor and if the resistor becomes too hot the control card stops the start up.

If many start ups are made during troubleshooting the resistor becomes too hot and the status code 51 will occur

NOTE: This status code -51 will often occur instead of
-151 when the fault is in the hydraulic pump controller. A fault in the hydraulic controller capacitor bank can be displayed as a fault in the traction controller capacitor bank (check both capacitor banks) as a -51 code will often be displayed before a -151

Check for open or loose connections in the capacitor bank.

Check for open control fuse

Replace control card

Hope this helps?
Shane