As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
FLIGHT 5378, STL TO PHL - JUST AFTER TAKEOFF CREW RECEIVED A LANDING GEAR AIR/GND FAIL MESSAGE AND THE LANDING GEAR LEVER WOULD NOT MOVE TO THE UP POSITION. THE AIRCRAFT RETURNED TO STL AND LANDED WITHOUT INCIDENT. UPON LANDING, THE AIR GROUND MESSAGE DISAPPEARED. MAINTENANCE REPLACED LGEU AND THE NOSE WEIGHT-ON-WHEELS PROX SWITCH ALONG WITH THE NOSE LANDING GEAR WEIGHT-ON-WHEELS PROX SWITCH HARNESS AND THE AIR/GROUND SYSTEM OPERATED NORMALLY.