How much does it cost to fix Vaillant error code F.23?

Typical repair cost for the Vaillant F.23 fault is £100-300 including labour.

F.23 High

Vaillant F.23 Fault Code

Q: What does the Vaillant F.23 fault code mean?

Your boiler is struggling to circulate water properly, often because the system pressure is too low or there is trapped air preventing the heat from moving away from the boiler.

Also written as F23 — same fault.

The Vaillant F.23 fault code means: your boiler is struggling to circulate water properly, often because the system pressure is too low or there is trapped air preventing the heat from moving away from the boiler.. It appears on 53 Vaillant models in our database.

Technical description: Water level low Temperature range too wide | Cause: Pump obstructed, pump running at lower capacity, Air in appliance, system pressure too low, flow and backflow NTC reversed

What causes the Vaillant F.23 fault?

This fault occurs when the boiler detects a massive temperature difference between the water going out to your radiators and the water coming back, suggesting the heat isn't moving. The most common reasons are that the system doesn't have enough water pressure to move or there is a large pocket of air trapped inside the internal pump. In some cases, the pump itself has reached the end of its life and can no longer push the water around your home effectively.

This fault often appears in autumn when the heating is turned on for the first time after summer, as pump components can seize during long periods of inactivity.

Typical repair cost £100-300 (including labour)

DIY fix possible

Check the pressure gauge and top up to 1.5 bar using the filling loop if it is too lowBleed your radiators to remove any trapped air from the systemEnsure all radiator valves are fully open and not stuck

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Vaillant models that show the F.23 fault code

Click a model for the full fault code list and model-specific repair notes.

Source

Vaillant F.23 fault code information on this page is sourced and cross-checked against the official Vaillant service / installation manuals for each affected model.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 27 April 2026