The boiler is struggling to detect when you turn on a hot tap, which may result in lukewarm water or no hot water at all.

What causes this fault?

This fault occurs when the internal sensor responsible for monitoring water movement fails to tell the boiler that you have opened a tap. The most common reason is that the small internal turbine has become jammed with limescale or debris, preventing it from spinning and sending a signal. In some cases, the electrical connections between this sensor and the main control board may have become loose or damaged.