Micheal M
I more or less sure that a code 15 means that the battery voltage is lower than the setting of the truck controller and 51 means your capacitor voltage is to low, I sugest you get your battery tested, if it checks out ok then you may need laptop service tool to alter power controller parametters. I relise this advice is abit late in coming to you, and you may have allready solved your problem.

Regards Titus