Heater display shows LO and will not fire; water is circulating but heater does not start.

Safety First: Turn off power to the heater before removing any panels. Do not run the heater with valves closed.

• Verify pump operation: Ensure the pool pump is running at normal speed and primed. Low pump speed or air in the pump basket can cause LO.
• Clean the filter: Backwash sand/DE filters or clean cartridge filters to restore proper flow. High filter pressure is a sign of restriction.
• Check valves: Confirm that inlet and outlet valves to the Hayward heater are fully open and that any bypass valve is correctly positioned per the installation manual.
• Inspect the water pressure switch: Locate the water pressure switch on the water manifold. Check for loose wires and ensure the tubing or port is not clogged with debris. If the switch does not close with good flow, it may need replacement.
• Test: Restore power, run the pump, and call for heat. If LO persists with clean filter and open valves, have a technician inspect the heat exchanger and hydraulic design.