BoilerFaultCodesVaillant F.23 Fault Code
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 45 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.
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.
- ecoFIT Pure 412 Repair: £150-300
- ecoFIT Pure 418 Repair: £150-350
- ecoFIT Pure 425 Repair: £150-350
- ecoFIT Pure 435 Repair: £150-350
- ecoFIT pure 612 Repair: £150-300
- ecoFIT pure 615 Repair: £150-300
- ecoFIT pure 618 Repair: £150-300
- ecoFIT pure 630 Repair: £150-300
- ecoFIT Pure 825 Repair: £150-300
- ecoFIT Pure 830 Repair: £150-300
- ecoFIT Pure 835 Repair: £150-350
- ecoFIT sustain 415 Repair: £150-350
- ecoFIT sustain 418 Repair: £150-350
- ecoFIT sustain 430 Repair: £150-350
- ecoFIT sustain 615 Repair: £150-£350
- ecoFIT sustain 618 Repair: £150-£350
- ecoFIT sustain 630 Repair: £150-£350
- ecoFIT sustain 825 Repair: £150-£350
- ecoFIT sustain 830 Repair: £150-£350
- ecoFIT sustain 835 Repair: £150-£350
- ecoMAX Repair: £150-450
- ecoMAX 613 Repair: £100-300
- ecoMAX 618 Repair: £100-300
- ecoMAX 622 Repair: £100-300
- ecoMAX 635 Repair: £100-300
- ecoMAX 824/2 E Repair: £100-300
- ecoMAX 828/2 E Repair: £100-300
- ecoMAX 835 E Repair: £100-300
- Ecomax Condensing 600/800 series Repair: £150-350
- ecoTEC exclusive Repair: £180-450
- ecoTEC plus Repair: £100-£350
- ecoTEC plus VU GB 656/4-5 A Repair: £180-350
- ecoTEC pro Repair: £150-£350
- ecoTEC sustain Repair: £100-£350
- Home Combi 25 Repair: £150-300
- Home Combi 30 Repair: £150-300
- Home Combi 35 Repair: £150-300
- Home Regular 12
- Turbomax 824/828 Repair: £150-300
- Turbomax Plus Repair: £120-250
- Turbomax Plus 824/828E Repair: £120-250
- Turbomax Plus 828 Repair: £150-£350
- Turbomax Plus 828 E Repair: £120-250
- Turbomax Plus VUW 242E Repair: £120-250
- Turbomax Plus VUW 242EH Repair: £120-250
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.
- Vaillant ecoFIT Pure 425 manual (PDF)
- Vaillant ecoFIT Pure 435 manual (PDF)
- Vaillant ecoFIT pure 630 manual (PDF)
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 31 August 2026