Zodiac pool heater shows E04 and shuts down; often occurs shortly after startup.

Safety: Turn off power and allow the heater to cool before touching internal components.

• Check water flow: Ensure the pump is running at the correct speed (for variable-speed pumps) and that filters and baskets are clean. Low flow can cause rapid temperature rise and trip the high limit.
• Inspect bypass and valves: Verify that any internal or external bypass is set according to Zodiac specifications. Too much bypass can reduce flow through the heat exchanger.
• Examine high limit switches: Locate the high limit switches on the header/manifold. Check for loose connections or signs of overheating. With power off, test continuity with a multimeter; a tripped or failed switch will be open.
• Descale heat exchanger (advanced): If the heater is older or used with hard water, internal scaling may be restricting flow. This typically requires a professional to chemically descale or replace the heat exchanger.
• Replace faulty high limit: If flow is correct and the switch tests open at normal temperatures, replace the high limit switch with a Zodiac-approved part.