F.035 Emergency

Vaillant F.035 Fault Code

Also written as F035 — same fault.

The Vaillant F.035 fault code means: your boiler is struggling to breathe because the pipe that vents exhaust gases outside or brings in fresh air is blocked or obstructed.. It appears on 3 Vaillant models in our database.

Act immediately

Turn off your boiler and stop using gas appliances. Call a Gas Safe engineer now. This fault can be dangerous if ignored.

Technical description: Air/flue pipe blocked during initial start-up. The air/flue pipe is blocked.
Typical repair cost £120-250 (including labour)

Engineer required

This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.

Find a Gas Safe engineer

Search the official register for a qualified engineer in your area.

Gas Safe Register →
Boiler cover options

Avoid the next call-out bill

A cover plan would handle faults like F.035: no call-out fees, no surprise bills.

Approved partner
Domestic Appliance Guard
Unlimited call-outs, 24/7 helpline, no price-hike promise
Get a quote →

Affiliate links. We may earn a commission at no extra cost to you.

Vaillant models that show the F.035 fault code

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

Source

Vaillant F.035 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 7 July 2026