Heater shows E09 and will not start or shuts down.

Safety: Turn off power before servicing.

• Locate inlet sensor: The inlet water temperature sensor is mounted on the inlet side of the heater header.
• Check connections: Inspect the sensor plug and wiring for corrosion, moisture intrusion, or mechanical damage. Dry and reseat connectors as needed.
• Measure resistance: With the sensor disconnected, measure resistance and compare to Pentair’s chart for the measured water temperature. Replace the sensor if out of range.
• Board diagnosis: If the sensor tests good and wiring is intact, the control board may be misreading the sensor and should be checked or replaced by a professional.