One thing to check because it's easy. There is a safety interlock switch that needs the battery side covers to be on for the truck to operate. Make sure that switch is closed. Located at the bottom where the cover slides in.

Is code 331 a current code? If it comes up every time you try to run then it could be the problem although usually traction codes don't affect hydraulics. If it's from a month back and not coming up every time then it's probably not the main problem.

I've had trouble with the distribution board on some of these trucks. It's located under the step for getting on the truck. You might want to check all connections and tap on the relays, check fuses etc.

I can't find a code 331. Sometimes they're not in the book though. I'd check the sensor wiring at the sensor and at the Access 3 panel. The red wire at the sensor is +12 V and goes through wire 312 to pin 1 on the panel. The black wire is battery negative and goes through wire 517 to pin 2 on the panel. The blue wire is Chanel B and goes through wire 015 to pin 14 on the panel. The white wire is Chanel A and goes through wire 014 to pin 13 on the panel.

Chanel A and B produce square waves that go up in frequency as the motor speed increases. The square wave would be measured between blue and black for channel B and between white and black for channel A.

It might be quickest to swap Access 3 panels if you have a sister truck. See if the problem follows the panel.

And I don't have to tell you to have the drive wheels off the ground right? If the forklift runs you over then they have to find someone else to fix the truck.
Likewise there's safety concerns when removing and installing the battery.