Won't start . Red light flashes twice. Says check psi , tube for water or blockage. Can't find tube their talking about.

This is the electrical area. You're better off posting here: Heating & Air Conditioning - Ask Me Help Desk

Depending on the efficiency of the unit, you may get water in the pressure sensor. Open the front of the heater and next to, or near the burner manifold should be a round device that looks like an old vacuum advance for a distributer. There is a plastic tube going into the plenum and 2 wires. Make sure the tube is open. Make sure there is no water inside the switch. You can also do a quick check of the switch by connecting the 2 leads together with a jumper wire and trying the heater. If the heater works, you need to replace the switch. HOWEVER, do not continue to run the heater without this safety. In addition, they may not close because the inducer fan is sensing back pressure from a blocked vent. Check that too, very carefully. If you are unsure of what you are doing, please consult a qualified service technician.