Controller displays HLO or HIGH LIMIT and shuts down solar heating; water circulation continues but panels are bypassed.

Safety: Turn off power to the Fafco controller before working on sensors or actuators. Use caution on roofs and ladders.

• Check setpoints: On the Fafco controller, verify the pool temperature setpoint and maximum roof temperature limit are correctly programmed per the manual. An overly low high-limit setting can cause nuisance HLO.
• Inspect roof sensor: Locate the Fafco roof (collector) sensor on or near the solar panel array. Ensure it is firmly attached, not in direct contact with hot metal, and wiring is intact. Replace if the sensor body is cracked or wiring is brittle.
• Inspect pool sensor: Check the pool water sensor at the plumbing manifold. Confirm it is fully inserted into the well or pipe saddle and that the cable is not damaged.
• Check solar valve actuator: With power restored, command solar ON and watch the Fafco solar valve actuator. If it does not rotate to send water to the panels, it may be stuck, causing overheating in a stagnant loop.
• Reset: After correcting issues, power-cycle the controller to clear HLO and monitor operation for at least one full heating cycle.