Furnace clean and tune, cycled system. Checked heat exchanger, found some rust, but no cracks at this time. Checked electrical system. Checked and adjusted gas pressures to maximize efficiency. Found drain trapped plugged. Removed and cleaned out trap. Water is now moving freely. Checked for levels of CO and XS air in exhaust, normal levels found. No CO found in airstream. System is operating normal at this time. Job complete.