Display shows F08; heater does not respond to thermostat changes.

Safety first: Turn off power before working on sensors or wiring.

• Locate the sensor: The water temperature sensor (thermistor) is typically threaded into the water manifold or installed in a sensor well near the heater inlet or outlet.
• Inspect wiring: Check the sensor leads for cuts, abrasions, or rodent damage. Ensure the connector at the control board is firmly seated.
• Compare displayed temperature: With the system off and water circulating, compare the heater’s displayed temperature to an accurate pool thermometer. A large discrepancy suggests a bad sensor.
• Resistance test (advanced): With power off and sensor disconnected, measure resistance across the sensor leads and compare to Aquacomfort’s temperature/resistance chart. If readings are far off, replace the sensor.
• Replace sensor: Shut off water and relieve pressure. Unscrew the old sensor, install the new one with appropriate thread sealant, reconnect wiring, and restore power. Verify that F08 clears and temperature reads correctly.