Free furnace safety check. Cycled system. Checked heat exchanger, some rusting, no cracks found. Checked and cleaned flame sensor. Checked igniter, some ware but tested ok. Checked gas pressure. Ok. Found indications of small water leak from ventor motor/ transition assembly. Not major at this time, but we will have to monitor it in future. Normal levels of CO and XS air found in exhaust. No CO found in airstream. System operating normal at this time. Job complete.