could be a bad PTC across the line contactor buss (that's the flat capacitor looking device that hooks between the buss bar connections on the line contactor, about the size of a quarter)
may be open, could have a bad connection.

also welded 1A contactor can cause this (if equipt with one)

check for continuity across the open 1A contactor

possible wiring error, insure no device is connected to the load side of the line contactor

worse case scenario is a bad controller unit