Your boiler has detected that the main part responsible for heating your water is becoming blocked or dirty and needs a professional service.

What causes this fault?

This fault occurs when the boiler detects a build-up of debris or mineral deposits inside the main heat exchanger, which restricts the flow of water. Over time, sludge from the central heating system or limescale from hard water coats the internal surfaces, making the boiler work harder and less efficiently. If left uncleaned, these deposits can prevent the heat from being transferred correctly, eventually leading to a full breakdown.