Vaillant water pressure sensor fault code is F75, which indicates the boiler isn't detecting a pressure rise when the pump starts up. This could be due to a blocked or faulty pressure sensor, a failing pump, low system pressure from a leak, or even issues with the boiler's printed circuit board (PCB). While you can try to top up the system's pressure and reset the boiler, if the F75 code persists, you should contact a Gas Safe registered engineer for diagnosis and repair. 

What is the F75 fault code?

• It's a safety measure:

• When the boiler starts, the pressure sensor should register an increase in water pressure from the pump. If it doesn't, the boiler shuts down to prevent potential damage. 

• It points to a specific problem:

• The F75 code specifically means there's an issue with the boiler's ability to read or detect water pressure changes. 

Common Causes

• Faulty or Blocked Pressure Sensor:

• The sensor itself might be blocked by debris (like magnetite) or be faulty, preventing it from accurately reading the pressure. 

• Failing Pump:

• The pump might be failing, intermittently seizing, or not circulating water correctly, leading to no detected pressure change. 

• System Leaks:

• Hidden or obvious leaks in the central heating system can cause low water pressure, triggering the fault code. 

• Low System Pressure:

• The water pressure in your central heating system might simply be too low and needs to be topped up. 

What You Can Do

• Check the Boiler's Pressure Gauge: Look at your boiler's pressure gauge to see if it's too low (usually below 1 bar).

• Top Up the System (if necessary): If the pressure is low, follow the instructions to top up the system's water pressure using the filling loop.

• Reset the Boiler: Attempt to reset the boiler by following the manufacturer's instructions for your model. 

When to Call a Professional

• If the F75 code returns:

• If the code reappears after topping up and resetting, the problem is more serious than just low pressure. 

• If there's a faulty sensor or pump:

• You'll need a qualified Gas Safe registered engineer to diagnose and replace the faulty component.