Gem won't go into reverse

Folks -
I have a GEM E2 with a T3 controller. The aftermarket motor on the car recently burned up, requiring me to replace it with the original 5hp GE motor. The car now runs fine in forward gear,but gives a 66 error code immediately upon any attempt to back up. Does anyone have any idea what might be the problem, and how it might be repaired?
Thanks in advance - Ed

The car reverses by reversing the field.

Control uses a separate set of transistors for forward and reverse.

IMO one or both of the rev. field transistors are shorted.

That was my guess. Where are the field transistors located, and are they easily replaced? If not -does the controller need a rebuild?
Best - Ed

Are you up for board level repairs?

I’ve had success with this. Change all four if you do it yourself.

No problem there. Are there schematics available, and part numbers?
Best - Ed

Not that I know of.
Parts have good markings to cross reference.
When you open control, it’s obvious which TO-220 devices connect to field terminals.
Will show dead short on bad one. Compare all four.
If no numbers, I’ll share the ones I use.

STP80NF 12
MOSFET N-ch 120V 80A TO-220

Received the parts from M*****, and replaced the bad transistor. I’m not receiving a -51 error, and when I check the voltage at the + and - end of the controller, I’ve no power. The fuses (particularly F13) are fine, and all 12 volt functions work. Any idea what else might be blown?
Thanks in advance - Ed

Still run forward?
If not, retrace your work. Something changed.
You won’t have power on B+ at control until master relay pulls in.
Controller logic must be happy for that to happen.