Furnace check. Cycled system. Removed burners and checked igniter/flame rod assembly. Both are very worn. Igniter has deep etch and crack forming across main face. Flame sensor is also worn. Burners are corroded and worn. Heat exchanger is rusted and corroded. It has deep etching in all chambers along with weak spots where cracks are forming. Checked gas pressures and adjusted to maximize efficiency. Found rust throughout both compartments. Found crack in transition assembly with residue leaking from current crack. Found CO and XS air higher than recommended, this indicates partial blockage of secondary heat exchanger. Currently no CO found in airstream. Average life expectancy of this unit is 14 years. This unit is 18 years old and worn. Due to age, condition, and cost of repair, second opinion is recommended. Future failure is expected. Job complete.