Your boiler has lost its ability to measure the temperature of the water leaving the unit, usually due to a faulty internal sensor or a loose wire.

Primary thermistor fault (Over-temperature sensed by primary sensor)

What causes this fault?

This fault occurs when the boiler's internal computer loses communication with the sensor that monitors the temperature of the water being sent to your radiators. The most common reason is that the sensor has physically failed due to age or internal wear, though it can also be caused by a loose electrical plug or a broken wire within the boiler.