Vaillant S.23 Fault Code

What causes the Vaillant S.23 fault?

This fault occurs when the boiler fails to establish or sustain a flame specifically while trying to heat your tap water. It is usually caused by a component failure that prevents the spark from jumping or the gas from flowing correctly, meaning the system cannot safely ignite to provide heat. The most common reasons include a worn-out ignition probe or a faulty gas valve that isn't opening as it should.