Technical description
The heating flow thermistor is short circuited.
What causes this fault?
This fault occurs when the sensor responsible for monitoring the temperature of the water leaving your boiler fails or develops an electrical short. The most common reason is that the sensor has reached the end of its life due to constant heat cycles, or moisture has compromised the electrical connection. Because the boiler can no longer accurately measure the water temperature, it shuts down to prevent the system from overheating.
Engineer required
This is a high severity fault. Stop using the boiler and call a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.