Your system has a faulty temperature sensor in the main pipework, which means it cannot accurately monitor the water heat flowing through your home.

Header Thermistor (T1) Error – for cascade and hybrid

What causes this fault?

This fault occurs when the main temperature sensor responsible for monitoring the shared water supply becomes unresponsive or provides an impossible reading. The most common reason is a physical failure of the internal sensing element, though it can also be triggered by loose or corroded electrical connections between the sensor and the main control board.