Furnace clean and tune. Cycled system. Checked heat exchanger, rusting. Checked and cleaned flame sensor and burners. Found igniter with wear. Checked gas pressure and checked for CO in airstream, none found. Normal levels of CO and XS air found in exhaust . System is older. Average life of this unit is 14 years. System showing signs of aging, but operating normal at this time. Job complete. Upgraded to pkg clean and tune.