Your boiler has a fault with the thermometer that measures hot water temperature, meaning your hot water may be unreliable although your heating should still work.

What causes this fault?

This fault occurs when the sensor responsible for monitoring the temperature of your tap water fails to send a consistent signal to the boiler's control board. The most common reason is that the internal thermistor has worn out over time or a small amount of corrosion has developed on the electrical connections. Without this data, the boiler cannot safely judge how hot to heat your water, leading to the error message.