Viessmann F4 fault codewhat it means and how to fix it
The boiler did not detect a flame and has locked out on the gas valve. It needs a Gas Safe registered engineer.
Stop — this needs an engineer
Get a Gas Safe registered engineer to look at your Viessmann F4
Gas Safe Register engineers only. No obligation.
What can you rule out from outside the boiler?
The right answer depends on what you can see. Pick the one that matches.
Choose the option on the left that matches what your appliance is doing, and the answer for that case appears here.
Other gas appliances have stopped as well
The supply is the problem, not the boiler.
Check the meter valve and prepayment credit, then contact your supplier.
It is below freezing
Frozen condensate is very likely, particularly on a long external pipe run.
Thaw with warm water along the pipe and reset once.
Neither of those applies
The fault is inside the appliance and needs qualified diagnosis.
Book a Gas Safe registered engineer and reset no more than twice meanwhile.
What F4 means
F4 is a flame failure lockout on the Vitodens range. The boiler runs its ignition sequence and monitors for a flame signal; without one, it closes the gas valve and latches the fault. The lockout prevents the appliance repeatedly introducing unburnt gas into the combustion chamber, which is why the code holds until it is deliberately reset rather than clearing on its own. Gas supply, the ionisation electrode and, in cold weather, a blocked condensate pipe account for most occurrences. Viessmann electrodes are a routine service item and their condition is one of the first things an engineer will check.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| No gas at the appliance | Meter credit, a closed service valve, or a supply interruption. | Common |
| Ionisation electrode worn or contaminated | The electrode no longer produces a reliable flame signal. A routine service replacement. | Common |
| Frozen or blocked condensate | Prevents the boiler completing a cycle, and presents as an ignition failure. | Common |
| Gas valve fault | Not opening within the ignition window, or opening too slowly to establish a stable flame. On a Vitodens this is the most expensive of the possibilities and the one an engineer will reach for last. | Occasional |
Can you fix it yourself?
Stop — this needs an engineer
What it costs to fix
Typical repair cost
£130–£480
Time on site
1h 15m
Ionisation electrode £130–£220 fitted. Gas valve £300–£480.
Typical UK prices including parts and labour, updated August 2026. Your quote may differ.
Parts you may need
We may earn a commission if you buy a part through one of these links. It does not change what you pay, and it does not affect which parts we list.
Condensate trap and siphon
COND-TRAP£15–£38Plumbing merchant
Price checked 5 August 2026
What other owners found
Nobody has shared what worked for them on this code yet. If you have had it, you would be the first — and it genuinely helps the next person.
Related Viessmann fault codes
- EA 227Stop — call an engineerThe boiler tried to light and never detected a flame, so it has shut the gas off and locked out. This is the most common Worcester fault code.Related: ignition and gas supply
- E01Get it checkedThe appliance tried to light and could not confirm a flame, so it has locked out. In winter a frozen condensate pipe is the most likely reason.Related: ignition and gas supply
- F.28Stop — call an engineerThe boiler tried to light and failed three times in a row, so it has shut the gas off and locked out. Do not keep resetting it.Related: ignition and gas supply
- 501Get it checkedNo flame was detected during ignition and the appliance has locked out. Check the gas supply and the condensate pipe before booking anyone.Related: ignition and gas supply
- A01Get it checkedThe burner failed to light and the appliance has locked out. Check the gas supply and, in cold weather, the condensate pipe before calling anyone.Related: ignition and gas supply
- L2Stop — call an engineerThe boiler failed to detect a flame during ignition and has locked out on the gas valve. Do not keep resetting it.Related: ignition and gas supply
Related guides
Common questions about Viessmann F4
What is the difference between F4 and F2 on a Viessmann?
F4 is a flame failure — no flame detected. F2 relates to a temperature limiter tripping. They are unrelated faults with unrelated fixes, so read the code carefully off the display rather than working from memory.
How often should the electrode be replaced?
Most engineers replace it as a matter of course every few years on an annually serviced boiler, and it is inexpensive as part of a service. Leaving it until it fails turns a £20 part into an emergency call-out in January.
Can I reset it myself?
Once or twice, yes, and if it clears and stays clear the cause was probably transient — a brief supply interruption or a condensate blockage that shifted. If it comes back, stop and call someone.
Sources and how we verified this
We establish the facts from the sources below and then write the explanation ourselves. We do not copy manufacturer wording and we do not host their manuals.
- Manufacturer technical and service documentation
- Manufacturer support portal
- Trade body guidance
Checked by Technical reviewer (Gas Safe registration to be displayed on appointment) on 5 August 2026.