First Alert FA120C - Manual (Page 27)

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TROUBLE CONDITIONS
3. A display of "BAT" with no zone number or beeping indicates that
the main standby battery in your control is weak. If this condition
persists for more than one day (with AC present), CALL FOR
SERVICE.
4. A display of "BAT" with a zone number and a once per 40
seconds "beeping" at the keypad indicates that a low battery
condition exists in the wireless sensor displayed.
Either replace the
battery yourself, or CALL FOR SERVICE. If the battery is not
replaced within 30 days, a CHECK display may occur.
Power Failure If there is no keypad display at all, and the POWER indicator (if
present) is not lit, operating power for the system has stopped and the
system is inoperative. CALL FOR SERVICE IMMEDIATELY.
If the message "AC LOSS" or "NO AC" is displayed, and the
POWER indicator (if present) is off, the keypad is operating on
battery power only. If only some lights are out on the premises, check cir-
cuit breakers and fuses and reset or replace as necessary. CALL FOR
SERVICE IMMEDIATELY if AC power cannot be restored.