The boiler saw no pressure change when the pump started, pointing to a faulty pressure sensor, pump or expansion vessel.
Most common on: ecoTEC plus, ecoTEC pro
F.75 means that when the pump started, the boiler did not detect the expected change in water pressure. The control board expects the pressure sensor to register the pump moving water; when it does not, it flags F.75.
It most often points to a faulty water pressure sensor, a failing or airlocked pump, or a waterlogged expansion vessel. It commonly appears alongside low-pressure symptoms, and it is not a homeowner fix — it needs an engineer to test the sensor, pump and vessel.
If the gauge reads low, top up to about 1.0–1.5 bar (cold) with the filling loop and reset — occasionally low pressure and air can bring on F.75.
Press reset after topping up. If F.75 clears and does not return, it may have been air or a transient glitch.
With the heating calling for heat, feel whether the pump is running and warm. A silent, cold pump may be seized or airlocked.
A recurring F.75 needs an engineer to test the pressure sensor, pump and expansion vessel and replace whichever has failed.
Work on the gas side of a boiler must be carried out by a Gas Safe registered engineer. The checks above are limited to safe, non-invasive things a homeowner can look at (pressure, resets, the gas supply) — if in doubt, stop and call an engineer.
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The boiler saw no pressure change when the pump started, pointing to a faulty pressure sensor, pump or expansion vessel. F.75 means that when the pump started, the boiler did not detect the expected change in water pressure. The control board expects the pressure sensor to register the pump moving water; when it does not, it flags F.75.
You can try a single reset and the safe checks below, but Vaillant F.75 usually needs a Gas Safe registered engineer. Never work on the gas side of a boiler yourself — it is illegal and dangerous unless you are Gas Safe registered.
A recurring F.75 means the underlying cause has not been fixed — most often faulty water pressure sensor. Resetting repeatedly will not solve it; get the boiler diagnosed properly.