Your system's main temperature sensor is sending incorrect information, which means your heating might not work efficiently or at all.

Header Thermistor (T1) Error (for Hybrid/Cascade)

What causes this fault?

This fault occurs when the sensor responsible for monitoring the temperature of the water entering your heating system fails or sends an impossible reading. The most common reason is a physical failure of the thermistor itself, though it can also be caused by loose wiring or a communication error within the main control board. Because the boiler no longer knows the exact temperature of the water, it shuts down as a safety precaution to prevent overheating.