What to do if AC INPUT FAULT is on for UTC Fire and Security Control Unit?
- WWilliam RomeroAug 9, 2025
If the UTC Fire and Security Control Unit displays an 'AC INPUT FAULT On' message, it indicates a loss or degradation of primary power. First, notify your electric company about the service loss. Then, check the connections to the Power Management Unit (PMU) Board TB1 and use a voltmeter to check for 120/240 Vac at TB1. After that, remove both secondary and primary power, disconnect all field wiring (labeling them first), and ensure the AC Selector Switch is correctly set for your line voltage. Finally, reinstall the board, connect the wiring, and reapply power. Perform functional tests to ensure proper operation.