There is an internal pressure switch for this unit that must sense water pressure before the gas valve solenoid is energized so that burner will light. If the air line to the pressure switch is leaking or has failed or is blocked it will not light at all. Additionally if you have had mice or smaller rats get into the unit, check the wiring to the pressure switch for any signs of damage. They can cause havoc! (Personal experience)

If you need further assistance please include your exact model number of your pool heater in your response.

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Kelly