I was called to a customer's house to repair a furnace that was not working. Upon arrival, I found that the furnace was not producing any heat. I checked the thermostat and found that it was set to "off." I reset the thermostat and tested the furnace again. This time, the furnace started to produce heat. I checked the burners and found that they were working properly. I also checked the air filter and found that it was dirty. I cleaned the air filter and tested the furnace again. This time, the furnace started to produce heat and the customer was happy.
