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Vaillant · Error F23

Vaillant F23 — what it means

The temperature gap between the flow and return is too wide — usually a circulation problem.

⚠ This is the likely cause from the code — a Gas Safe registered engineer confirms it on site. If you can smell gas, stop and ring 0800 111 999.

✅ Try these first — they're free

Often clears it without a callout:

  • Try resetting the boiler once (your manual shows how)
  • If the code comes straight back, it needs a Gas Safe engineer — don't open the boiler yourself

🔧 If that doesn't fix it

The likely cause is one of these — the engineer confirms which:

    Most likely part: Pump or circulation fault

    Fixed repair price
    £456
    Inc. VAT — part & labour, no surprise callout fee. You only pay if you go ahead.
    A vetted Gas Safe engineer who covers your postcode will call you back to arrange the repair. No callout fee — you only pay if you go ahead.

    Other Vaillant fault codes

    Vaillant F0A flow temperature sensor has lost its signal to the boiler.…Vaillant F1A return temperature sensor has lost its signal to the boile…Vaillant F10The flow temperature sensor has short-circuited.…Vaillant F11The return temperature sensor has short-circuited.…
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