Safety first: Turn off heater power before working on the circulation system.

Verify circulation: Confirm the pool pump is running and the pump basket is clean.
Clean filter: Backwash or clean the cartridge/DE filter to restore flow.
Check valves and bypass: Ensure valves to and from the heater are open and any bypass is correctly set to send adequate flow through the heater.
Inspect inlet screen: Some Jandy heaters have an inlet screen or strainer at the header; clean any debris.
If FLO persists: The flow or pressure switch may be defective and should be replaced.

Display shows FLO or similar flow-related message; heater will not fire.

Possible Causes