Your boiler is currently unable to read the water temperature correctly because of a faulty internal sensor connection, which has caused it to stop heating your home as a safety precaution.

Return sensor short circuit.

What causes this fault?

This fault occurs when the boiler's internal computer detects a broken electrical signal from the sensor that monitors cooled water returning to the unit. The most common reason is that the sensor's internal components have failed or the wiring connecting it to the main control board has become damaged or loose.