Vaillant F.28 fault codewhat it means and how to fix it
The 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.
Stop — this needs an engineer
Get a Gas Safe registered engineer to look at your Vaillant F.28
Gas Safe Register engineers only. No obligation.
What else in the house uses gas?
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.
The gas hob and other gas appliances have also stopped
The boiler is fine — there is no gas getting to the property. Check the emergency control valve near the meter is in line with the pipe, and check a prepayment meter for credit.
If the meter has credit and the valve is open, ring your gas supplier. This is not a boiler fault.
Other gas appliances work normally
Gas is reaching the property, so the fault is inside the boiler — most likely the ignition or flame sensing electrode.
Book a Gas Safe registered engineer. Reset the boiler no more than twice while you wait.
It happened during a hard frost
A frozen condensate pipe is very likely. The white plastic pipe running from the boiler out through an outside wall backs up with ice and the boiler locks out.
Thawing the external section with warm — not boiling — water often clears it. If the boiler then lights and stays lit, that was your fault.
You can smell gas
Stop. Do not reset the boiler, do not touch any electrical switch, and do not look for the leak yourself.
Open the windows, leave the property, and call the National Gas Emergency Service on 0800 111 999 from outside. It is free and staffed around the clock.
What F.28 means
F.28 is an ignition lockout at startup. The boiler runs its normal sequence — fan up to speed, gas valve open, spark — and then looks for a flame signal on the sensing electrode. If it does not see one, it shuts the gas valve, purges, and tries again. After three failed attempts it locks out and displays F.28. The lockout exists to stop the appliance repeatedly filling a combustion chamber with unburnt gas. That is why the reset limit matters: each reset restarts the whole three-attempt sequence, and forcing it over and over is how you get an unburnt gas build-up in an appliance that has already told you something is wrong.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| No gas reaching the boiler | A closed service valve, an emptied prepayment meter, or a tripped emergency control valve. Check whether other gas appliances in the house still work. | Common |
| Failed ignition or flame sensing electrode | The electrode either does not spark or, more often, sparks fine but no longer detects the flame it has lit. Electrodes carbon up and their gap widens with age. | Common |
| Faulty gas valve | The valve does not open, or opens too slowly for the ignition window. A genuine engineer diagnosis, not a guess. | Occasional |
| Condensate pipe frozen | A blocked condensate causes a range of lockouts in cold weather, and on some installations it presents as a failed ignition rather than a dedicated condensate code. | Occasional |
Can you fix it yourself?
Stop — this needs an engineer
What it costs to fix
Typical repair cost
£130–£420
Time on site
1h 15m
Electrode replacement £130–£220. Gas valve replacement £280–£420. If gas supply is the problem, your supplier deals with it at no cost to you.
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.
Vaillant ecoTEC ignition and sensing electrode
0020039221£19–£46Authorised Vaillant parts distributor
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 Vaillant fault codes
- F.29Stop — call an engineerThe boiler lit correctly and then lost the flame while it was running. It has shut the gas off and locked out, and it needs an engineer.Related: ignition and gas supply
- F.62Stop — call an engineerThe gas valve is not shutting off when the boiler tells it to. This is a gas safety fault and the boiler must not be used until an engineer has seen it.Related: ignition and gas supply
- F.61Get it checkedThe gas valve control circuit has faulted. The boiler will not fire and this is firmly an engineer's job.Related: ignition and gas supply
- F.27Stop — call an engineerThe boiler is detecting a flame when the burner should be off. Switch it off and get a Gas Safe engineer out rather than resetting it.Related: ignition and gas supply
- F.54Get it checkedA gas supply fault, usually shown alongside F.28 or F.29. It means gas is reaching the appliance at too low a pressure.Related: ignition and gas supply
- 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
Related guides
Common questions about Vaillant F.28
How many times can I reset an F.28?
Twice, at most. Each reset triggers three more ignition attempts, and each failed attempt puts a small amount of unburnt gas into the combustion chamber before the valve closes. If two resets have not cleared it, the fault is real and needs an engineer, not a third attempt.
Is F.28 dangerous?
The lockout is the boiler protecting you and it is doing the right thing. The danger comes from overriding it repeatedly. Leave it locked out and call someone qualified.
Why does it light in the morning but not in the evening?
Intermittent ignition faults usually point to a marginal electrode or a gas supply that struggles under load. It is a fault getting worse, not a fault that comes and goes for no reason, and it will fail completely soon.
What does an electrode replacement cost?
Typically £130 to £220 fitted, and it is usually a single-visit job. The electrode itself is inexpensive; most of the cost is the call-out and the combustion check afterwards.
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
- Trade body guidance
- Interviews with working engineers
Checked by Technical reviewer (Gas Safe registration to be displayed on appointment) on 5 August 2026.