Vaillant F.28 fault codewhat it means and how to fix it
The boiler tried to light and failed every attempt it is allowed — three on an ecoTEC, five on the 2022 CS/1-5 — so it has shut the gas off and locked out.
Stop — this needs an engineer
Get a Gas Safe registered engineer to look at your Vaillant F.28
We do not pass enquiries to a repair firm, so there is no form here. Find a Gas Safe registered engineer through the Gas Safe Register, and ask to see their registration card when they arrive — it is the register, not the van, that tells you they are qualified.
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. How many tries it gets is published per range: the ecoTEC plus, pro, exclusive, ecoFIT and ecoTEC sustain operating instructions all say the boiler "switches to fault mode" after three unsuccessful attempts, the ecoMAX user guide says five, the turboMAX plus guide gives it as ignition not achieved within ten seconds, and the three-digit ecoTEC plus CS/1-5 shows F.028 "after five unsuccessful ignition attempts".
Whatever the count, 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 sequence, Vaillant cap it at three resets before you call someone, 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. The published check list is longer than most people expect.
The ecoTEC plus table says the gas pressure monitor or the thermal cut-out has been triggered and sends the engineer through the gas stopcock, gas flow pressure, gas valve assembly, air intake pipe, condensate route, plugs and cable harness, ignition transformer, cable, plug and electrode, monitoring electrode, electronics, earthing and the CO₂ setting — so an F.28 is a symptom with a dozen candidates, not a diagnosis of the electrode. The CS/1-5 has a sibling with no two-digit ancestor: F.281, "the flame has failed during the stabilisation period" — a flame that lit and went out again in the seconds after ignition — which its operating instructions treat exactly as F.028, with the same five attempts, the same gas stopcock check and the same three-reset limit, so it is covered here rather than on a page of its own.
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 September 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 28 September 2026
Own this appliance and had this code? . Nothing is published until a person has read it.
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 flame
- 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 flame
- F.68Get it checkedThe flame is lighting but its signal is unstable. Vaillant's causes run through the gas supply, the air ratio, the condensate route and the flue, ending with the electrode and the board.Related: ignition and flame
- F.87Get it checkedThe ignition transformer, or on the ecoFIT pure the electrodes, are not connected as the board expects. A wiring fault on the ignition side; the boiler will not attempt to light.Related: ignition and flame
- F.22Get it checkedThe boiler has shut down because there is not enough water in the system. Pressure has dropped below roughly 0.5 bar and it will not fire until you top it up.Related: same brand
- F.75Get it checkedThe boiler cannot detect a rise in pressure when the pump starts, so it has locked out. Usually a failed pressure sensor or a pump that is not moving water.Related: same brand
Own a Vaillant boiler? Save it and its codes come first next time.
Related guides
Common questions about Vaillant F.28
How many times can I reset an F.28?
Vaillant's own limit is three: "if you have been unable to eliminate the ignition fault after three fault clearance attempts, consult a competent person", and the CS/1-5 instructions print "maximum number of repetitions: 3". Each reset triggers another full run of ignition attempts, and each failed attempt puts a small amount of unburnt gas into the combustion chamber before the valve closes. If the second reset has not cleared it, the third is unlikely to, and the fault needs an engineer.
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.
Technical review
Not reviewed by an engineer
No engineer has read this page. Severity follows what the manufacturer publishes, and we would rather tell you that than let a byline imply a sign-off.
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
This page has not been reviewed by an engineer, and we would rather tell you that than imply otherwise. The severity rating follows what the manufacturer publishes: where they say isolate the appliance and call someone, so do we.