As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
SEA - AT FLIGHT LEVEL 34,000, AFT CARGO DOOR WARNING ILLUMINATED FOLLOWED BY A PROX SYSTEM 2 FAULT MESSAGE. MAINTENANCE INSPECTED THE DOOR AND SURROUND WITH NO DEFECTS NOTED. OPERATIONAL CHECK OF THE AFT CARGO DOOR GOOD. AIRCRAFT RETURNED TO SERVICE. NO PARTS REPLACED.