I was called to a home to repair a split Trane furnace that was not working. I arrived at the house and found that the furnace was not producing any heat. I checked the thermostat and found that it was set to "off." I checked the power supply and found that it was working fine. I checked the gas valve and found that it was open. I checked the igniter and found that it was working fine. I checked the flame sensor and found that it was working fine. I checked the blower motor and found that it was working fine. I checked the air filter and found that it was dirty. I cleaned the air filter and found that the furnace was now producing heat. I tested the furnace to make sure it was working properly and found that it was. I gave the customer a receipt for the service and explained to them how to maintain the furnace in the future.