I also have some notes that indicate that there is a possibility that some RF "noise" created (induced) by having some of the (shielded) control cables routed to close to the power cables may cause some of these codes, and have a note that adding a capacitor (like is between the connections on the coil of some contactors) mounted between the key on positive and the frame of the truck can reduce or eliminate the incidence of code 66 w.i.f.