Heat tune up for 120KBTU natural gas furnace: Replaced filter. Made call for heat from thermostat/verified thermostat operation. Visual inspection of burners and heat exchanger. Checked hot surface ignitor ohms and looked for cracks/hot spots during start up. Cleaned flame sensor. Hacked flame sense/rectification circuit. Checked all electric motors and components. Checked temperature rise and static pressure. Checked gas pressure. Combustion draft test: Good Furnace is operating well at the time of this maintenance.