Furnace clean and tune. Cycled system. Checked heat exchanger, found both shots are very rusted and worn. Cracks starting to form. Checked and cleaned flame sensor and burners, both are very worn and damaged. Checked igniter, found worn. Checked for CO and XS air in exhaust, normal levels found of both. No CO found in airstream. Average life of this unit is 14 years. This system is 20 years old and worn. Failure is expected. Due to age, condition, and cost to repair, second opinion is recommended. Job complete.