As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
IAD TO GSO - AFTER TAKEOFF, WHEN THE CREW SELECTED THE LANDING GEAR UP, A LANDING GEAR AIR/GROUND FAIL MESSAGE WAS DISPLAYED ON EICAS AND THE LANDING GEAR WOULD NOT RETRACT. THE CREW RETURNED TO FIELD AND LANDED WITHOUT INCIDENT AT IAD. MAINTENANCE TROUBLESHOT THE AIR/GROUND SYSTEM AND FOUND THE LEFT MAIN LANDING GEAR WEIGHT-ON-WHEELS PROX SWITCH FAULTY. AFTER REPLACING THE PROX SWITCH, THE MESSAGE CLEARED AND THE AIR/GROUND SYSTEM AND LANDING GEAR OPERATED NORMALLY.