first off a full serial number of the truck in question would be a great help.
Also if this truck happens to have the EV100 control system on it, going by the error codes your problem is relatively simple.

code -50 indicates 'low capacitor volts' and the truck will have no travel function. Your problem with this code pertains to the #2 Rec either being faulty or a bad connection to it. The code is triggered when the #2Rec fails to turn on at initial startup.

code -70 indicates 'Regen current sensor input missing' (yellow wire on sensor circuit)
check your circuit on the sensor for open circuit or loose connection between sensor and PA4 plug on controller card.

If you remove the PA4 connector at the card and take some needlenose pliers (small ones) and go in and put a slight twist on the pins this will insure a positive connection at the connector terminals, they are known to become loose and cause poor connections at the plug triggering all sorts of random codes. And when checking the SCR's (#2Rec) also check the #5Rec as well as all your contactor tips. Sticky contactor tips or dirty contactors will cause problems too.

Also make sure battery is fully charged, low voltage can cause random codes to be triggered.