Your boiler is struggling to accurately measure the temperature of the water returning to it, which causes it to stop operating to stay on the safe side.

Return sensor out of range. Sensor measures a temperature outside normal working range.

What causes this fault?

This fault occurs when the sensor responsible for monitoring the water returning to the boiler sends a signal that is outside of its expected safety limits. The most common reason is that the sensor has developed an internal electrical fault or the wiring connecting it to the main control board has become damaged. In some cases, it can also be triggered if the water inside the system reaches an extreme temperature that the sensor is unable to process.