Furnace clean and tune. Cycled system. Checked heat exchanger, some rust but no cracks found. Checked burners and flame sensor, cleaned and checked. Checked gas pressures and adjusted to maximize efficiency. Checked for XS air and CO in exhaust, found normal levels . No CO found in airstream. Replaced filter, office to schedule ac clean and tune in spring. Paid. System operating normal at this time. Job complete.