Paloma Tankless water heater wont kick on

Question

I have model PH-12M-DN the pilot light will come on and stay light but when I turn the water on to demad the unit kick on it does not...I am guessing maybe a bad pressure switch but have no manual for this and can't find one on line. The unit is about 10 years old but is used in my poohouse and is on only half the year at most..It has worked perfectly every time I have turned it on in the past..any suggestions ?