Unlike a low-pressure error (E37) or a radiator needing bleeding,
The fault code appears on Vaillant ecoTEC plus, pro, and exclusive series boilers. It indicates a flame detection error — specifically, the boiler’s electronic control unit detects a flame signal when the burner should be off. f64 vaillant
This is a very common cause. The ionisation electrode (or flame sensor) is responsible for detecting whether a flame is present. If this sensor is dirty, misaligned, or faulty, it sends incorrect data to the main board, triggering the F64 error. Unlike a low-pressure error (E37) or a radiator