Display shows FS and heater will not start or shuts down shortly after starting.

Safety first: Turn off power at the breaker and verify the control is dead before touching wiring.

• Locate the water temperature sensor: On AquaCal units, the sensor is typically threaded into the water manifold or installed in a sensor well on the inlet/outlet plumbing.
• Inspect wiring: Check the sensor cable from the probe to the control board for cuts, chew marks, or UV cracking. Ensure the plug is fully seated on the control board terminals.
• Clean connections: If there is corrosion on the connector, disconnect power, unplug the sensor, and clean contacts with electrical contact cleaner. Reconnect firmly.
• Compare displayed temperature: Restore power and compare the displayed water temperature to an accurate pool thermometer. If the reading is wildly inaccurate or FS persists, the sensor is likely bad.
• Replace the sensor: Shut off water flow, relieve pressure, and remove the old sensor from the manifold. Install the new AquaCal‑compatible water temperature sensor with thread sealant approved for plastics/metal, then reconnect wiring.