Furnace clean and tune. Cycled system. Checked heat exchanger. Found all chambers were rusting and corroded. Did not find any cracks with camera. Checked,more,over, and cleaned burners and flame sensor. Both ok. Igniter tested normal. Checked and adjusted gas pressures to maximize efficiency. Checked and tightened electrical connections and clamps. Cleared drain lines. Preformed combustion test. Normal levels found in exhaust. No CO found in airstream. Average life expectancy for this unit is 14 years. Future failure is expected. Removed old thermostat and installed new T-6 WiFi stat. System operating normal at this time. Job complete.