Furnace repair. No heat. I found a faulty pressure switch that monitors the pressure created by the inducer fan during startup. It ensures the furnace doesn't operate unless proper venting is established, preventing dangerous backdrafting or gas leaks. If the switch doesn't detect the correct negative pressure, it will shut down the furnace. I installed a new pressure switch. The 20-year-old furnace is working now. Will send an estimate to replace it with a new energy-efficient furnace.