I was called to a home to check on a furnace that was not working. Upon arrival, I found that the furnace was working great with no issues found. I tested the thermostat and found that it was working properly. I also checked the air filter and found that it was dirty and needed to be replaced. I cleaned the air filter and tested the furnace again. The furnace was working great and the air was warm and comfortable. I made sure to leave the customer with instructions on how to maintain the furnace and check the air filter regularly.