I was called to a home to check out a furnace that was not working. I arrived at the house and found that the furnace was working great with no issues found. I tested the furnace and found that it was working properly. I also checked the thermostat and found that it was working properly. I made sure that the furnace was properly installed and that the vents were clear. I also checked the air filter and found that it was dirty and needed to be replaced. I replaced the air filter and tested the furnace again. The furnace was working great and the customer was happy.