The calibration button on the CO2 Gas Sensor can become stuck in the depressed position by being caught under the edge of the plastic. If this has happened, the red LED will not flash and the sensor will not calibrate when the button is pressed. This can usually be fixed by using the tip of a stylus or paper clip to gently press and move the button slightly to the side, freeing it from being caught under the plastic. If this does not work, please call Vernier’s tech support for assistance (888-837-6437).