I did call a Yale dealership and it's about $500 for them just to show up, so I'm going to keep dinking around with this. Yale said it's very weird symptoms. He also didn't have an answer for me about a 36v system/battery having 48v controllers, but the motors say 36 or 48v.

I'm having a really hard time finding any troubleshooting online for this model, so I'm going to keep posting my progress here in hopes that someone has some insight into it.

The batteries seem fine according to my charger. Everything works crisp and like new with the pump motor. Power steering, lift, tilt, etc. There are no codes thrown. I was mistaken on the codes, it was just simple stuff: E-brake warning, not in seat, pressing accelerator when not in fwd/rev.

The main issue is travel/traction motor speed. It only will creep along. Barely moving. There was one moment when it took off intermittently, but I haven't been able to get it to repeat that.

The cables from the black box controller to the motor get warm/hot when I try to travel. I lifted the battery box out and inspected the motor connections hoping for massive corrosion, but there was none. I pulled the wires off the motor, cleaned the connections with a wire brush and reassembled with no improvement.

I also checked the accelerator pedal potentiometer (if that's the right name) and it changed resistance as I moved the pedal. Not sure the right way to test, but it seems to work.

I'll just keep researching and trying stuff until I fix it or take it in.
Thanks,
Kevin