Your boiler's internal temperature sensors are struggling to read the water temperature correctly, preventing the system from heating up properly.

Sensor fault

What causes this fault?

This fault occurs when the boiler's internal thermometers, known as NTC sensors, stop communicating accurate temperature data to the main control board. The most common reason is that the sensors have physically degraded over time or have become coated in internal system debris, meaning they can no longer tell the boiler if the water is hot or cold. As a safety precaution, the boiler stops firing because it cannot be sure it won't overheat.