Heater displays E05 and shuts down; may occur during operation after running for some time.

Safety first: Turn off power to the heater and allow it to cool. Do not touch hot plumbing or metal parts.

Step-by-step checks:

• Inspect outlet sensor: Locate the outlet temperature sensor (NTC) on the heater outlet header or pipe. Check that it is firmly inserted and clamped in place, with good thermal contact.
• Check wiring harness: Follow the sensor cable back to the control board. Look for cuts, pinched sections, or corroded connectors. Reseat the connector on the board.
• Measure resistance: With power off and wires disconnected, measure the resistance of the outlet sensor using a multimeter. Compare the reading to the Pahlen temperature/resistance chart at the current water temperature. Replace the sensor if readings are out of range or intermittent.
• Check for localized overheating: If the sensor is good, inspect the outlet header for scale or partial blockage that could cause hot spots. This may require removing unions and visually inspecting the header and tubes.

When to stop: If you cannot access or test the sensor safely, or if the heat exchanger appears scaled, contact a pool heater technician.