As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
BUF/PHL - FLT 871 - AFTER TAKEOFF, GEAR WOULD NOT RETRACT AND GEAR WARNING INDICATED. AT 400 FEET 'AIR/GND SENSOR' WARNING APPEARED ON MFDU. FLIGHT RETURNED TO BUF WITHOUT FURTHER INCIDENT. AIRCRAFT FERRIED GEAR DOWN TO PIT FOR MAINTENANCE. MAINTENANCE REPLACED LEFT INBOARD MAIN GEAR PROXIMITY SENSOR DUE TO BEING BROKEN. GEAR SWING PERFORMED. OPERATIONAL CHECK GOOD. (X)