The F64 error code on a Vaillant boiler indicates a problem with the boiler's temperature sensor or the thermistor. This error is often associated with issues related to the NTC (Negative Temperature Coefficient) sensors, which monitor the temperature of the water in the boiler.
Do not ignore this error or attempt to bypass it. Contact a qualified heating engineer to inspect the gas valve, electrodes, and electronics. f64 vaillant
The main control board is unable to process sensor data correctly. The F64 error code on a Vaillant boiler