As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
(CAN) AFTER TAKEOFF UNABLE TO POWER AC ESSENTIAL BUS EITHER BY AC BUS NR 1 OR AC BUS NR 2 AIRCRAFT RETURN TO TORONTO. FOUND TO BE THE ESSENTIAL BUS SENSE RELAY AT FAULTY.REPLACED AC ESSENTIAL RELAY, POWER AVAILABLE FROM BOTH BUSES. OPERATIONAL TEST 24-51-27-710-002 CARRIED OUT.