What it means
What F4 is telling you
F4 means the Vitodens attempted ignition and its ionisation probe never saw a flame, so it locked out rather than continuing to release gas.
The three usual causes are no gas at the burner, a dirty or degraded ionisation electrode, or a condensate blockage — the same family of causes as a Worcester EA.
In cold weather, check the condensate first. A frozen condensate pipe is responsible for a large share of the no-flame lockouts we attend every January.
Before you call
Safe checks you can do yourself
- Check other gas appliances work — no gas anywhere means it's a supply issue
- Thaw any frozen external condensate pipe with warm, not boiling, water
- Check the gas meter isolation valve is fully open
- Reset once; if it locks out again, book an engineer
Never remove the boiler casing or work on the sealed combustion chamber yourself — by law that’s Gas Safe registered engineers only. If you smell gas, call 0800 111 999 immediately.
How we fix it
What we do on the visit
- Gas working pressure and gas rate tested properly at the appliance
- Ionisation electrode cleaned or replaced and correctly gapped
- Burner and combustion chamber inspected and combustion analysed
- Condensate route cleared, re-pitched or insulated to stop it recurring
How urgent is it?
No heating or hot water. Same-day appointments available across our patch.
Costs
What a repair costs
Our diagnostic visit is £99 inc VAT and it comes off the bill if we carry out the repair. We give you a fixed price before touching anything — no hourly rates, no surprises on the invoice.
We’re Gas Safe registered, Which? Trusted Trader endorsed and rated 4.9 across 206+ reviews. We carry the common Worcester Bosch and Viessmann spares on the van, so most faults are fixed on the first visit rather than booked back in.
Common questions
How much does a Viessmann F4 repair cost?
Diagnostic is £99 inc VAT, refunded against the repair. Most F4 faults are an electrode clean or replacement, typically £99–£240 inc VAT completed.
Why does my condensate pipe keep freezing?
Usually because it runs outside with too little fall, too long a run, or no insulation. We re-pitch, upsize or lag it so you're not repeating this every cold snap.
Can I clean the electrode myself?
No — it's inside the sealed combustion chamber and only a Gas Safe registered engineer may open that. Doing it yourself is both illegal and dangerous.
Book it in
Get an engineer out
Step 1 / 5
60 second boiler quote
No callbacks unless you want one
What can we help with?
Pick the option that fits — we'll tailor the next questions.
More help
Other fault codes
Worcester Bosch
EA
No flame detected — ignition lockout
Worcester Bosch
E9
Overheat — the boiler has got too hot
Worcester Bosch
A1
Pump fault or poor water circulation
Worcester Bosch
C6
Fan fault — fan speed out of range
Worcester Bosch
D5
Control board or communication fault
Viessmann
F2
Temperature limiter — overheat lockout