Your boiler is simply performing a routine internal check-up to ensure everything is working correctly and will resume normal operation shortly.

Vaillant self-test. The Vaillant self-test is activated.

What causes this fault?

This code appears when the boiler’s internal computer runs a temporary diagnostic check to verify that all sensors and safety components are communicating correctly. It is not triggered by a broken part, but rather a routine automated sequence that briefly pauses heating activity. Once the boiler confirms all systems are healthy, the code will clear and the unit will fire up as normal.