As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
AIRCRAFT EXTERNAL POWER WOULD NOT TURN ON AIRCRAFT POWER. LOCATED AND REPAIRED OPEN WIRE W218-035-10 AT TERMINAL LUG TO T7 OF EXTERNAL POWER T/R UNIT ON E1-4 SHELF, TERMINAL WAS BURNT. FOUND DEFECTIVE EXTERNAL POWER CONTACTOR NR 1 RELAY R-56, REF WIRING DIAGRAMS 24-141-12 AND 24-41-11. REPLACED EXTERNAL POWER CONTACTOR.