As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
ORD - FLT 6021 THE CREW REPORTED AN APU FIRE MESSAGE ON THE GROUND AT THE GATE. MAINTENANCE INSPECTED FOR SIGNS OF FIRE. NO FUEL OR OIL LEAKS WERE NOTED AND NO BURNING OR SMOKE SMELLS WERE FOUND. FOUND APU INLET WAS CLOGGED WITH ICE. REMOVED ICE AND PERFORMED OPERATIONAL CHECKS AND NO DEFECTS WERE FOUND.