As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
LEFT BLEED DUCT WARNING ILLUMINATED AT APPROXIMATELY FL 270. WITHIN A MINUTE WARNING CHANGED TO LEFT PACK FAIL. CABIN CLIMBED TO FL 180, PAX OXYGEN MASKS AUTOMATICALLY DEPLOYED. DECLARED EMERGENCY AND RETURNED TO FIELD. REPLACED MT 139 LOOP A & B AND SUPPORT STAND OFF FOR LOOPS.