What does Vaillant F35 mean?

The air or flue pipe is blocked.

How do I fix Vaillant F35?

Try first: Check the outside flue and condensate pipe aren't blocked or frozen; Press the reset button once. If that doesn't clear it, the likely part is Flue or condensate blockage.

Vaillant F35 Fault — What It Means & Fix Cost

⚠ 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:

Check the outside flue and condensate pipe aren't blocked or frozen
Press the reset button once

🔧 If that doesn't fix it

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

Most likely part: Flue or condensate blockage

What it costs

Fixed price,
no callout fee

Free diagnosis first, then one fixed price for the repair — 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.…

Vaillant F35 — what it means

✅ Try these first — they're free

🔧 If that doesn't fix it

Other Vaillant fault codes

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