Vaillant [F.73] fault codewhat it means and how to fix it

F.73 and F.22 are easily confused and the difference is worth getting right. F.22 means the appliance has read the pressure and found it too low. F.73 means the appliance cannot get a sensible reading from the sensor at all — Vaillant describe it as the water pressure sensor signal being out of range at the low end, caused by a broken cable, a short circuit to 0V, or a defective sensor. In other words, F.22 is about the water and F.73 is about the measurement. That distinction has a practical consequence: on an F.73 the pressure gauge will very often read perfectly normally, and topping the system up will change nothing at all. People frequently refill two or three times before working that out. The good news is that a pressure sensor and its wiring are among the cheaper repairs on a Vaillant, and the fault is straightforward for an engineer to confirm with a multimeter.