Vaillant F.25 Fault Code

What causes the Vaillant F.25 fault?

This fault occurs when the boiler's safety sensors detect that the internal temperature has risen too quickly or that the electrical connection to the thermal module has been broken. The most common reason is a loose or damaged wiring harness, but it can also be triggered if the heat exchanger becomes too hot due to a lack of water flow. To prevent damage, the system automatically shuts down until the electrical circuit is restored or the unit cools down.