Furnace locks out with E06, often when there is a demand for domestic hot water (if combi) or during heating.

Safety: Disconnect power before removing covers.

• Locate return NTC: The return temperature sensor is typically mounted on the return pipe entering the furnace or on the lower part of the heat exchanger.
• Check plug and wiring: Confirm that the sensor connector is fully engaged and that the cable is not pinched or cut where it passes through the casing.
• Visual inspection: Look for corrosion or water leaks around the sensor pocket. Any leak must be repaired before replacing the sensor.
• Reset and test: After reseating the connector, restore power and reset the unit. If E06 reappears, the sensor or control board requires professional diagnosis and replacement.