F3 High

Viessmann F3 Fault Code

The Viessmann F3 fault code means: your boiler is detecting a flame when there shouldn't be one, so it has shut down for safety purposes to prevent a malfunction.. It appears on 9 Viessmann models in our database.

Call an engineer soon

Book a Gas Safe engineer within 24 to 48 hours. Your boiler may be unsafe or could break down completely if left.

Technical description: Burner in fault position. Flame signal is already present at burner start. Check the ionisation electrode and lead.

What causes the Viessmann F3 fault?

This fault occurs when the boiler's sensors detect a flame signal before the ignition sequence has even started. The most common reason is a build-up of carbon or moisture on the sensing probe which sends a false electrical signal to the control board, or a faulty internal component that incorrectly thinks the gas is already burning.

This issue is more frequent during periods of high humidity or heavy rain, as moisture ingress can affect the electrical signals on the ignition leads.

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 F3: 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.

Viessmann models that show the F3 fault code

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

Source

Viessmann F3 fault code information on this page is sourced and cross-checked against the official Viessmann service / installation manuals for each affected model.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 7 July 2026