Heater will not start; display shows F01 shortly after call for heat.

Safety first: Turn off power to the heater at the breaker and shut off gas before opening any panels. Do not run the heater with no water flow; this can damage the heat exchanger.

• Check pool circulation: Ensure the pool pump is running at normal speed and is fully primed. Verify that skimmer and pump baskets are clean.
• Inspect valves: Confirm that all valves on the suction and return lines are fully open and that any heater bypass valve is not diverting too much water away from the heater.
• Clean the filter: If the filter pressure is high or flow seems weak, backwash a sand/DE filter or clean cartridge elements. Restore normal system pressure and flow.
• Check heater inlet screen: Remove the heater’s water inlet union and inspect any internal screen or strainer for debris. Clean and reinstall, ensuring O-rings are seated properly.
• Test the flow switch: Locate the water flow switch/pressure switch on the water manifold. With power off, disconnect its leads and check continuity with a multimeter while the pump is running. It should read closed (continuity) with good flow. If it remains open, adjust the switch per the service manual or replace it.
• Reset and test: Restore power and gas, restart the pump, then call for heat. If F01 persists after confirming strong flow and a working flow switch, contact a qualified pool heater technician.