F6 error, water heater stops, temperature readings on display are unrealistic or fluctuating

Safety first: Turn off power and close water valves if you will open any hydraulic components. Allow hot water circuit to cool.

• Locate DHW NTC: Find the domestic hot water temperature sensor on the outlet pipe or in a sensor well on the heat exchanger.
• Inspect wiring: Check the sensor plug and cable for damage, corrosion, or loose connections at the main PCB.
• Measure resistance: Disconnect the sensor and measure its resistance at room temperature. Compare with Protherm's NTC chart. If reading is open, shorted, or far from expected, replace the sensor.
• Check sensor seating: Ensure the sensor is fully inserted into its pocket and has good thermal contact (use thermal paste if specified by Protherm).
• Reset and test: After replacement, clear the error by cycling power and run hot water to confirm stable temperature readings.

Warning: Do not bypass the NTC sensor; this can cause overheating and scalding risk.