Arrived and furnace would not ignite. Igniter would glow, but gas valve would not receive power to open. Found that plugged drain pressure switch was open. Blew through drain and found large blockage in drain line. Cleared drain line and tested operation. Performed annual Furnace precision tune up; clean and tune up furnace; clean burners, test ignition components, adjust gas level, test safety circuit, check indoor motors, checked all electrical components, tested indoor air flow, checked heating temperatures, and tested overall furnace operation. Main flame safety was failing. Replaced under parts only warranty and tested new part. Furnace now operating normal at this time.