As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
AT FL390 ACM OVER HEAT ANNUNCIATOR ILLUMINATED AND CABIN STARTED TO CLIMB. EMERGENCY CHECKLIST WAS FOLLOWED. EMERGENCY WAS DECLARED AND AIRCRAFT DIVERTED. LANDING WAS UNEVENTFUL. MAINTENANCE REPLACED THE ACM OVER HEAT SWITCH. A SUCCESSFUL OPERATIONAL CHECK FLIGHT WAS PERFORMED AND AIRCRAFT WAS RETURNED TO SERVICE.