The heater will click on in the start mode but will not come on. The ignighter does not light and the red light is flashing TWO Flashes. My owners manual says "presure switch is open" but does not say what presure switch or where it is. I would like to know where it is and how to check it? It also does not have this fault all of the time and when it does, I can shut the power off to the heater for a minute or so and it will work fine for a few days.