Vaillant F28 Fault Code Explained
F28 on a Vaillant boiler means ignition failure: the boiler went through its start-up, tried to light, and did not establish a flame, so it locked out. Like Ideal's L2 and Worcester's EA, it is a safety lockout rather than a danger in itself. The cause is often outside the boiler, such as no gas reaching it or a frozen condensate pipe, and if it keeps returning after a reset the fault is inside and needs a Gas Safe engineer.
The short answer
F28 = ignition failure. The fan ran, the gas valve opened and the boiler sparked, but no flame was confirmed, so the gas was shut off and the boiler locked out. Check that gas is reaching the house and the condensate pipe is not frozen, reset once, and call an engineer if F28 comes back.
What you can safely check
| Check | How | What it tells you |
|---|---|---|
| Gas to the house | Try the gas hob | If nothing lights, it is the supply, not the boiler |
| Prepayment meter | Check credit and emergency credit | No credit means no gas, and F28 |
| Meter control valve | Lever should run in line with the pipe | Across the pipe means the gas is off |
| Condensate pipe | Check the outside section in frosty weather | A frozen pipe locks the boiler out; thaw with warm water |
| Reset once | Follow your manual's reset procedure | If F28 returns, stop and call an engineer |
Common causes inside the boiler
If gas is reaching the house and the condensate is clear, the cause is inside the appliance. The usual suspects on an ignition failure are a worn or dirty ignition electrode, a flame sensing electrode that cannot detect the flame, a damaged ignition lead, low gas pressure at the boiler, a gas valve not opening properly, a fan or air supply problem, or a blocked flue. Diagnosing these properly means gas pressure readings, a flue gas analysis and testing components one at a time, which is gas work for a Gas Safe registered engineer only.
F28 and F29
Vaillant uses a separate code for a flame that lights and then goes out during operation. The distinction is useful: F28 is a failure to light at all, so the checks above, especially gas supply, come first. Your boiler's manual lists the exact codes for your model, and the same code can mean different things on different boilers, so check it before anyone orders a part.
The same fault on other makes
| Make | Code | Meaning |
|---|---|---|
| Vaillant | F28 | Ignition failure |
| Ideal | L2 | Ignition lockout (see what L2 means) |
| Worcester Bosch | EA | Loss of flame / no flame detected |
Preventing F28
Most ignition failures that are not about the gas supply come from wear that a service catches: electrodes that have eroded or carbon-coated, a condensate trap that is part-blocked, or a flue that has picked up debris. An annual service checks and cleans these, measures the gas pressure at the boiler and analyses the combustion, so a slow decline is caught before it ends in a lockout on a cold morning. Where the condensate pipe runs outside and has frozen before, having it upsized, lagged or re-routed stops the winter F28 for good.
Getting it fixed
An F28 caused by the gas supply or a frozen pipe costs nothing to put right. One caused by a worn electrode is a routine repair, and an annual service checks and cleans the electrodes so it is less likely to happen. On an older Vaillant with a failing gas valve or control board, the repair cost is worth comparing with replacement; we fit the Vaillant ecoTEC pro and ecoFIT pure, and our Vaillant boilers page lists the prices. Book a boiler repair and the engineer will give you the cost before any work.
Common questions
What does F28 mean on a Vaillant boiler?
Ignition failure: the boiler tried to light but did not establish a flame, so it shut the gas off and locked out. Check the gas supply and condensate pipe, reset once, and call a Gas Safe engineer if it returns.
How do I reset a Vaillant F28 fault?
Follow the reset procedure in your boiler's manual. Reset once; if the boiler lights and stays lit, the cause was probably temporary. If F28 returns, stop resetting and have it diagnosed.
Can a frozen condensate pipe cause F28?
It can cause the boiler to lock out in cold weather. If F28 appears on a frosty morning, check the outside section of the condensate pipe and thaw it with warm, not boiling, water from the ground.
Is F28 dangerous?
The lockout is a safety feature: the boiler has shut off its gas. If you smell gas, though, open windows, avoid switches, leave the property and call the National Gas Emergency Service on 0800 111 999.
Updated 15 October 2026
