Batteries Thermometer Look to see if the battery light is flashing on the thermostat. If it is, or if the display doesn't seem to be working, open the thermostat and replace the batteries. Hold a thermometer right beside the thermostat and take the air temperature. It should be within a couple degrees of the room temperature indicated on the thermostat. If it isn't, the thermostat's temperature sensor isn't working right. Adjust the temperature for the heater to above the current room temperature and check to see if the vents come on. If the vents do not come on, have a friend stand by the furnace. Turn off the heater, then turn it on again. Your friend should hear a loud click from the relay after you turn the thermostat on. If there is no sound of a relay clicking, the thermostat is probably broken. Repeat steps 3 and 4 for the air conditioning if you have an air conditioner. Be sure to use an accurate thermometer. Otherwise, you may think your thermostat is bad but have a defective test thermometer instead.