I am a professional HVAC technician with Dor-Mar Heating & Air Conditioning. I was called to a customer's home to check their heating system. I arrived and found that the furnace was not working. I diagnosed the issue as a faulty thermostat. I replaced the thermostat and tested the system. The furnace was now working properly. I also checked the air filters and found that they were dirty. I cleaned the air filters and tested the system again. The air filters were now clean. The customer was happy with the service and thanked me for my help.