To reset the Kidde 21029902 Combination Smoke and Carbon Monoxide Detector after it goes off and stops on its own, follow these steps: Press the Test/Reset Button: Locate the test/reset button on the alarm and press it. This will reset the unit to its normal standby mode. Check for Residual Smoke or CO: Ensure there are no lingering smoke or carbon monoxide particles in the area, as these could trigger the alarm again. Clean the Unit: If the alarm continues to go off unnecessarily, clean it with compressed air to remove any dust or debris that might interfere with the sensors.