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.74 is the other end of the pressure-sensor range fault: the signal is shorted onto 5 V or 24 V, or the sensor has an internal fault. System pressure may look normal on a mechanical gauge.
Vaillant meaning: Water pressure sensor signal outside range (high side / 5 V–24 V short)
Possible causes
- Sensor cable shorted to 5 V or 24 V
- Internal sensor fault
- Pinched loom after a pump or hydroblock change
Possible solutions
- Inspect the three-core sensor lead for trapped insulation, especially where it crosses the pump and case.
- Unplug the sensor. If the code changes to F.73, the sensor is likely shorting internally — replace it.
- If F.74 remains with the sensor unplugged, suspect the loom or PCB.
Possible parts needed
- Water pressure sensor 0020059717 (confirm GC)
- Harness
- PCB
