Furnace clean and tune. Cycled system. Checked heat exchanger. Medium rusting. No cracks found at this time. Igniter ok. Cleaned and checked flame sensor. Cleaned burners. Checked for CO in airstream, none. Normal levels of CO and XS air found in exhaust. System would not fire after I reassembled it. Found that the flame sensor was too close to burners. Adjusted and system cycled normal. System is operating normal at this time. Job complete. Office to bill. .