Vaillant F.75 Fault: No Pressure Change When Pump Starts — Causes & What To Do

F.75 indicates the boiler's pressure sensor didn't register a change when the pump started, so it can't confirm water is circulating. It locks out to protect the heat exchanger from overheating. You'll usually have no heating or hot water.

The most common causes, in order of how often we see them:

• • Faulty pressure sensor — the sensor that detects the pressure change when the pump runs can fail, so the boiler thinks nothing is circulating even when it is. The most common F.75 cause.
• • Seized or sticking pump — if the pump isn't turning properly, no pressure change is generated and the sensor reads nothing.
• • Air in the system — trapped air can prevent a clean pressure change at the sensor, triggering F.75 intermittently.
• • Low system pressure — very low pressure can mean the pump has nothing to push, so no change is detected.
• • Blocked pump or filter — debris in the pump or system can stop circulation.

Repressurising to 1.2 bar and resetting can clear F.75 if low pressure or air was the cause. If pressure is normal and the fault persists, the issue is the pressure sensor or the pump — a reset won't help. Don't reset repeatedly; if it returns, book an engineer.

• • Check the pressure gauge — if it's below 0.6 bar, low pressure may be your F.75 trigger. Repressurise to 1.2 bar via the filling loop, then reset.
• • Bleed radiators to release trapped air — air can prevent a clean pressure change at the sensor. Re-check pressure afterwards, as bleeding lowers it.
• • Listen for the pump — when the boiler tries to fire, you should hear the pump run. If it's silent or humming loudly, the pump may be seized and needs an engineer.
• • If pressure is normal, radiators are bled, and F.75 persists, the fault is the pressure sensor or pump — book a Gas Safe engineer.

The pressure sensor and pump sit inside the boiler casing. Replacing the sensor, freeing the pump, or any work inside the casing is gas/electrical work that must be done by a Gas Safe Registered engineer.

• • Confirm system pressure on the gauge and with a calibrated reading.
• • Test the pressure sensor's response when the pump starts.
• • Inspect the pump — check it's running freely and not seized or blocked.
• • Bleed the system and check for air locks affecting the sensor reading.
• • Test the PCB pump and sensor signals to isolate the fault.
• • Replace the pressure sensor or pump as needed, then re-commission.

Our Gas Safe Registered (594547) engineers repair every Vaillant model — ecoTEC, ecoFIT, atmoTEC — across Surrey, Berkshire, Buckinghamshire and West London, and carry common sensors and ignition parts on the van. Most Vaillant fault-code repairs are completed in a single visit, with any part quoted and agreed before fitting.