What does the 'No AC' or 'AC Loss' display mean, and what should I do?
The 'No AC' or 'AC Loss' display means the system is operating on backup battery power due to an AC power failure. If only some lights are out on the premises, check circuit breakers and fuses and reset or replace as necessary. If AC power cannot be restored and a 'low system battery' message also appears, you should call for service. (Page 39)
From the official ADT ADT 3000 Security System Keypad manual, page 39
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