If you are currently without heating and hot water due to this fault, contact a local Gas Safe engineer to diagnose the specific component failure and restore your system safely.
Unlike low water pressure errors, the F64 fault code There are no DIY fixes for this specific error because it involves the gas train and high-voltage electronics. vaillant boiler f64 fault
However, If the fault returns immediately or within a few minutes, there is a hardware failure that requires professional diagnosis. Continual resetting of a gas valve fault can be dangerous. If you are currently without heating and hot
Here is the direct explanation and what you should check: vaillant boiler f64 fault