Event code 201 means the access 2 is receiving voltage that is too high or to low. Sometimes a fuse can go bad without blowing. Check resistance on the fuse attached or before the access 2 module.

To check if a battery is bad, measure the battery voltage idle, then put a load on the battery and measure that voltage. If it drops below the rated battery voltage (ex. 36v), the battery might be bad.
To check if a battery is bad you can also measure voltage across each cell individually. If there is a bad cell it will have a drastically different voltage compared to other cells under load. Normally I see voltages like 2.10v, 2.11v, 2.10v, etc. Then one random cell goes down to 1.86v. That is the bad cell. That cell sucks power out of the other cells to try and get back to 2.10v resulting in the incorrect voltage being sent to the truck.