agreed..can be tough code to troubleshoot, especially without access to manual, training to understand components and their interaction. When the code appears though, can give good clues as to where to look first. If key-on self test code appears, usually looking at "hard" failure ie: TM 1 or 2 blown open transistor, failure of HCM, blown FU8, complete open of field power cable. On the other hand if it occurs during travel or intermittant, think, loose connection of field poer cables, loose connection of the t1,t2,t3 or t4 jumpers on field wiring board, or TM 1 or 2 failing under load or getting hot.
It should go without saying though that testing the battery and performing a frame ground test MUST be performed on any electric truck before troubleshooting further.