reset. We decided to call the fire depatment just to be safe and they confirmed that the alarm was failing and needs replacement. On most CO alarms, there is a tag on the back or inside the battery compartment which will have a date of manufacture or an expiration date (as shown w/ pencil in photo). Always keep the instructions in an accessable place for quick reference if an alert should sound, or to confirm when yours needs replacement. Carbon Monoxide is a real hazard and if you have a gas burning appliance in your home you definitely should have at least one CO detector.