Furnace clean and tune. Cycled system. Found system would fire and then drop out after a few seconds. It did this several times. Removed and cleaned flame sensor, and reinstalled. Cycled system. Found system was holding. Checked heat exchanger, some rusting. Checked igniter, found signs of it being worn, but tested ok. Checked and cleaned burners, ok. Checked and adjusted gas pressures to maximize efficiency. Checked for CO and XS air in exhaust. Normal levels found. No CO found in airstream. System is worn, but operating normal at this time. Recommend yearly clean and tune. Job complete.