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Raymond KlineAug 14, 2025
How to fix AC INPUT FAULT on Kidde Fire Systems Control Unit?
- PperezdavidAug 14, 2025
If the AC INPUT FAULT is on for your Kidde Fire Systems Control Unit, it could be due to a loss or degradation of primary power. First, notify your commercial light and power company about the service loss. Then, check the connections to the Power Management Unit (PMU) Board TB1, using a voltmeter to ensure you're getting the nominal 120/240 Vac at TB1. If this doesn't resolve the issue, remove both secondary and primary power, disconnect all field wiring (after labeling them), and ensure the AC Selector Switch on the power supply unit is correctly set for your line voltage. After checking the switch, reinstall the board, reconnect the field wiring, and reapply power. Finally, perform functional tests to confirm the system is working.