As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
KMDW - KMDW - DURING CLIMB WHILE PASSING 13,000 FT, THE EMERGENCY PRESSURIZATION SYSTEM ENGAGED. THE CREW DONNED THEIR OXYGEN MASKS AND RETURNED THE AIRCRAFT TO KMDW. THE CABIN AIR WAS REPORTED TO BE VERY WARM PRIOR TO THE EMERGENCY PRESSURIZATION SYSTEM ENGAGING. MAINTENANCE REPLACED THE 410 DEGREE SWITCH. BOTH OPERATIONAL AND LEAK CHECKS WERE GOOD. THE AIRCRAFT WAS RETURNED TO SERVICE.