Boiler
F04
The F04 error code on a Viessmann boiler indicates a fault with the boiler’s ignition — specifically that the burner has failed to light or has lost its flame signal. This is a lockout fault, meaning your boiler has shut itself down as a safety measure and will not restart without intervention.
What does this error mean?
When your Viessmann boiler displays F04, it has detected that the burner failed to ignite successfully or that the ionisation electrode — the component that confirms a flame is present — did not register a stable flame within the allowed timeframe. The boiler locks out automatically to prevent unburned gas from accumulating, which is a built-in safety response. Until the underlying cause is resolved, the boiler will remain locked out and will not provide heating or hot water.
Est. repair cost
£80–£250 for a Gas Safe engineer callout and repair (parts dependent); ignition/ionisation electrode replacement typically £80–£140 all-in
🔍 Likely causes
🛠️ Step-by-step fix
Check your gas supply first: confirm other gas appliances (hob, oven) are working. If they aren’t, contact your gas supplier — this is a supply issue, not a boiler fault.
If you have a prepayment gas meter, check there is sufficient credit and that the meter has not gone into emergency mode.
Press the reset button on your Viessmann boiler — typically a flame symbol button held for 3 seconds. Consult your manual for the exact location on your model (e.g. Vitodens 050-W, 100-W, 200-W).
Wait 10 minutes, then attempt one more reset. If the boiler lights and runs normally for a full heating cycle, monitor it closely over the next 24 hours.
If the F04 returns after one or two resets, do not attempt further resets — repeated failed ignitions can indicate a gas or component fault. Stop here and call a Gas Safe registered engineer.
Do not attempt to inspect, clean, or replace the ignition electrode, ionisation electrode, gas valve, or PCB yourself. These tasks legally require a Gas Safe registered engineer.
📞 When to call a professional
Call a Gas Safe registered engineer if the F04 error returns after one or two resets, if you can smell gas at any point, or if the boiler has been locking out repeatedly over recent days. When you call, tell the engineer you have a Viessmann boiler displaying a persistent F04 ignition lockout fault — this helps them bring the right parts and diagnose the job faster.
⚖️ Repair or replace?
A professional repair for an F04 fault typically costs £80–£250 depending on the faulty component — electrode replacements sit at the lower end, a gas valve or PCB at the higher end. If your Viessmann boiler is under 10 years old and has been well serviced, repair is almost always the right call. If it is over 12–15 years old and has had repeated faults, a new A-rated condensing boiler (£1,800–£3,500 installed) will be more cost-effective in the long run.