F8 displayed, unit shuts down, often accompanied by high outlet temperature

Safety: Turn off power and close water isolation valves. Allow hot water lines to cool before touching the outlet sensor.

• Locate DHW sensor: On Viessmann water heaters, the DHW NTC is typically clipped to or inserted into a sensor well on the hot water outlet pipe or plate heat exchanger.
• Inspect wiring and connector: Disconnect the sensor plug and check for moisture, corrosion, or loose pins. Dry and clean as needed, then reconnect firmly.
• Measure sensor resistance: With a multimeter, measure the NTC resistance at room temperature and compare to the Viessmann chart. Replace the sensor if readings are out of specification or if the sensor is open/shorted.
• Check for scale: If the sensor is in a well, remove it and inspect for limescale buildup. Clean the well gently and ensure good thermal contact when reinstalling.
• Reset and test: Restore power, clear the F8 code, and run hot water while monitoring outlet temperature. If the fault returns, the control board input may be defective and should be checked by a technician.