The furnace was not working properly. I checked the thermostat, and the batteries were dead. I replaced the batteries and tested the furnace. It was working properly at this time. I also checked the air filter and found that it was very dirty. I cleaned the air filter and tested the furnace again. It was working properly. I also checked the blower and found that it was not working properly. I replaced the blower and tested the furnace again. It was working properly. I also checked the gas valve and found that it was not working properly. I replaced the gas valve and tested the furnace again. It was working properly. I also checked the pilot light and found that it was not lit. I lit the pilot light and tested the furnace again. It was working properly.