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First Alert CO400B feature an electrochemical CO sensor, an 85dB horn a silence feature and a red LED that blinks to indicate unit is receiving power. The front access battery door allows battery access without removing the unit from the wall. The low power warning indicator will cause the alarm to "chirp" when the batteries need replacing. The low power warning indicator can be silenced for up to 8 hours. An Alarm Action Chart identifies action steps to take if unit is in alarm.