Furnace clean and tune. Cycled system. Checked heat exchanger. Found a lot of rust throughout heat exchanger, no cracks found at this time. Checked flame sensor and burners, bother we’re very corroded. Cleaned both. Igniter tested lower than expected, but still within range, worn. Checked and adjusted gas pressures to maximize efficiency. Checked for CO and XS air in exhaust. Normal levels found. No CO found in airstream. Found bearings in ventor motor are worn. System is worn, but operating normal at this time. Job complete.