Furnace locks out with E06, domestic hot water (if equipped) overheats or is cold, temperature readings fluctuate.

Safety: Disconnect power before removing any sensor covers or accessing the control board.

• Identify which sensor: Consult the Viadrus manual to see whether E06 refers to the return NTC or DHW NTC on your specific model.
• Visual inspection: Locate the relevant NTC sensor on the return pipe or DHW cylinder pocket. Check for damaged insulation, signs of overheating, or water ingress at the cable entry.
• Test resistance: Disconnect the sensor and measure its resistance at room temperature. Compare to the Viadrus NTC chart. Replace the sensor if readings are out of range.
• Check wiring route: Ensure the sensor cable is not routed alongside mains cables or pump power cables, which can induce noise. Re‑route if necessary and secure with clips.
• Replace sensor: If faulty, install a genuine Viadrus NTC sensor of the same rating. Insert fully into the pocket and secure with the clip or screw provided.