Your boiler is providing a status update to show it is currently heating up the water for your taps.

Burner start up in hot water mode

What causes this fault?

This code appears when the boiler detects a demand for hot water and begins the ignition sequence to heat it. It is not actually a fault, but a status message indicating that the internal burner is firing up to provide water for your taps or shower. Once the water reaches the required temperature, the code will usually change or disappear.