Your boiler is simply letting you know that the 'warm start' feature is active, which keeps a small amount of water pre-heated so you get hot water at your taps more quickly.

Warmstart demand

What causes this fault?

This fault occurs when the boiler attempts to ignite to provide hot water but fails to detect a consistent flame or correct operation. The most common reason is a communication error between the sensors and the burner, often triggered by a faulty diverter valve sticking or a sensor incorrectly reporting water flow. It essentially means the boiler’s safety controls have stepped in to stop the burner because it cannot guarantee correct operation in hot water mode.