Gas Safe: Gas work, flue work, and PCB replacement on a gas boiler must be carried out by a Gas Safe registered engineer. Homeowners can check obvious items such as heating pressure, condensate overflow, and whether a control is unplugged — then call an engineer.
Always confirm the part number against the boiler GC number and the installation manual before ordering.
Overview
F.73 is an electrical fault on the water-pressure sensor circuit: open, short to ground, or a sensor that is outputting too low. It is not simply “low system pressure” (that is F.22).
Vaillant meaning: Water pressure sensor signal too low / out of range (low)
Possible causes
- Pressure sensor unplugged or open circuit
- Short to GND on the sensor supply/signal
- Defective pressure sensor
- Damaged loom
Possible solutions
- Check heating pressure is actually on the gauge (around 1.0–1.5 bar cold). Fill if empty, but F.73 usually remains if the sensor circuit is dead.
- Refit the sensor plug. Inspect for water inside the connector.
- Engineer check: sensor supply and signal on the PCB connector (see also F.75 voltage notes).
- Replace the sensor (later black type supersedes the old orange). Clean the finger filter in the hydroblock while it is off.
Possible parts needed
- Water pressure sensor 0020059717 (common later ecoTEC type — confirm GC)
- Sensor O-ring / clip
- Harness
- PCB if supply is missing
More on the sensor: Vaillant water pressure sensor.