Your boiler has detected that a key temperature sensor has failed or disconnected, which is preventing the system from heating up safely.

Primary thermistor open circuit.

What causes this fault?

This fault occurs when the boiler's main control board loses communication with the temperature sensor that monitors the heat exchanger. The most common reason is that the internal sensor component has failed due to age, or the electrical wiring connecting it to the brain of the boiler has become loose or damaged.