Your boiler is simply preparing to heat your tap water and is currently checking that its internal fan is spinning correctly before firing up.

DHW mode: Fan start-up. The fan start-up for domestic hot water mode is activated.

What causes this fault?

This code appears when you turn on a hot tap and the boiler begins its safety checks before igniting the burner. It happens because the control board is verifying that the fan is spinning at the correct speed to safely expel exhaust gases. As long as the code disappears and hot water follows, it indicates the boiler is functioning exactly as it should.