As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
UPON START UP OF THE APU THE CABIN PAC WENT TO FULL HOT IN AUTO MODE. SWITCHED TO MANUAL MODE AND TEMP RAN AWAY TO 180 DEGREES WITH BLUE HAZE IN CABIN. REMOVED AND REPLACED CABIN ACM PACKAGE BYPASS VLV AND DUCT TEMP SENSOR. OPS AND LEAK CHECK GOOD. ALL WORK ACCOMPLISHED IAW MM 21-21-00.