Furnace clean and tune. Cycled system. Checked heat exchanger. All chambers are showing signs of rust and corrosion. No visible cracks found with camera. Checked, removed and cleaned burners and flame sensor. Both ok. Checked and adjusted gas pressures to maximize efficiency. Preformed combustion test and found some indications of some blockage in secondary heat exchanger. No CO found in airstream. Blower motor has vibration, bearings are getting worn. System is operating normal at this time. Job complete.