Your boiler is simply performing a routine internal check as it starts up and the code should disappear after a few seconds.

Part of the self-test sequence sequence when the appliance is switched on.

What causes this fault?

This code appears when the boiler is first powered on or restarted, serving as a sign that the internal control board is running a diagnostic check. It occurs as the software verifies that all safety sensors and electronic components are communicating correctly before allowing the burner to light. It is not a malfunction, but rather a necessary step in the appliance's normal startup routine.