Vaillant S.5 Fault Code

What causes the Vaillant S.5 fault?

This status occurs when the boiler's internal computer is waiting for the flame sensors to confirm the burner has successfully lit. It usually signals a breakdown in communication between the ignition components and the control board, often caused by worn-out spark electrodes or a faulty gas valve failing to release fuel. If the boiler cannot confirm a safe flame after several attempts, it will eventually stop trying and display a full fault code.