As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
FLIGHT 7847, IAD TO YYZ - AFTER TAKEOFF, WHEN THE CREW ATTEMPTED TO RAISE THE LANDING GEAR THEY RECEIVED A LANDING GEAR AIR/GROUND FAIL MESSAGE ON EICAS AND THE GEAR HANDLE WOULD NOT MOVE TO THE UP POSITION. THE FLIGHT RETURNED TO IAD AND LANDED WITHOUT INCIDENT. MAINTENANCE INSPECTED AND TESTED THE PROX SENSORS AND FOUND THE AFT WEIGHT ON WHEELS PROX SENSOR ON THE LEFT MAIN LANDING GEAR WOULD NOT TEST. MAINTENANCE REPLACED THE FAULTY PROX SENSOR AND THE AIR/GROUND SYSTEM OPERATED NORMALLY.