Furnace clean and tune. Customer stated that furnace would continue to start up and shut down. I cycled system and found that the pressure switch would not stay closed. I checked drain lines and found them full of water. Cleared all drain lines and also pressure switch tubes. Cycled again and the switch is operating normal. Checked heat exchanger, some rusting. Flame sensor and burners were checked and cleaned. Igniter is showing some signs of being worn, but operating normal at this time. Normal levels of CO and XS air in exhaust. No CO found in airstream. System is operating normal at this time. Job complete. Ac and furnace clean and tune.