Vaillant F.28 Fault Code
The F.28 fault means the boiler attempted to ignite but failed to establish a flame. It is one of the most common Vaillant faults — and one of the most misdiagnosed. Here is what it means and how it is fixed.
What is the Vaillant F.28 fault code?
The Vaillant F.28 fault code indicates an ignition failure: the boiler's control board triggered an ignition sequence, but the flame detector (ionisation probe) did not confirm that a stable flame was established within the required time window. The boiler locks out as a safety measure and displays F.28. This can be a single component failure or a combination of issues.
What causes the F.28 fault?
If the gas meter or supply valve is closed, or a prepayment meter has run out, the boiler cannot establish a flame. Check other gas appliances before calling an engineer.
Dust, debris, or corroded burner ports can prevent a proper flame from forming. Common on boilers that have not been serviced in 2+ years.
The electrode that creates the spark to ignite the gas degrades over time. A cracked ceramic insulator or corroded tip prevents a consistent spark.
The high-voltage lead connecting the control board to the electrode can crack or arc to ground, preventing the spark reaching the electrode.
If the probe is coated with oxidation or combustion deposits, it cannot detect the flame even if ignition is successful. Common on older ecoTEC units.
A sluggish or partially failed gas valve may not open fully on demand, resulting in insufficient gas pressure at the burner during ignition.
What you can safely check yourself
Turn on another gas appliance (hob or gas fire). If nothing works, call your gas supplier — the issue is with the supply, not the boiler. For prepayment meters, top up and wait 10 minutes before resetting the boiler.
Look at the boiler's pressure gauge. It should read between 1.0 and 1.5 bar when cold. Low pressure (below 0.5 bar) can occasionally trigger an F.28 on some Vaillant models — repressurise via the filling loop and reset.
Press and hold the reset button (flame symbol) for 3–5 seconds. Allow a full minute for the ignition attempt. If F.28 returns immediately, do not reset again — call an engineer.
Do not reset the boiler more than once if the fault code returns immediately. Repeated resets without diagnosis can mask a worsening fault.
What needs a Gas Safe engineer
- Inspect and clean the burner and combustion chamber
- Test ignition electrode spark output with a multimeter
- Check and replace the ignition lead if arcing or cracked
- Clean or replace the ionisation probe
- Measure gas pressure at the inlet and manifold
- Test gas valve operation and replace if sluggish
- Check flue for blockages that can cause gas to back-fill and prevent ignition
In South London — including Wandsworth, Merton, Lewisham, Lambeth, Southwark, Bromley, and Greenwich — hard water accelerates carbon deposits on the ionisation probe and burner. A boiler that previously faulted with F.28 every 2–3 years may start doing so annually once limescale buildup affects heat distribution. Annual servicing is strongly recommended.
How much does a F.28 repair cost?
Costs below are UK averages. Reion provides a fixed quote after the £80 diagnostic visit — no obligation to proceed.
How to reset a Vaillant boiler showing F.28
Ensure the boiler has been off for at least 5 minutes before attempting a reset.
Press and hold the reset button — usually marked with a flame icon or the letter "R" — for 3 to 5 seconds.
Release the button and allow the boiler to complete its ignition cycle (up to 60 seconds).
If the display shows "F.28" again within 2 minutes, do not attempt another reset.
Call a Gas Safe engineer — repeated failed resets can indicate a worsening component fault.
If the boiler is over 12–14 years old and the gas valve or heat exchanger is the diagnosis, the cost of repair may approach or exceed the cost of a new boiler. Reion will provide an honest comparison during the diagnostic visit.
Common questions about the Vaillant F.28 fault
What does a Vaillant F28 fault code mean in simple terms?
Can a Vaillant F28 fault fix itself?
How long does a Vaillant F28 repair take in London?
Is it safe to use the hot water while the boiler shows F28?
Why does my Vaillant F28 fault keep coming back?
Other Vaillant fault codes
Need a Vaillant engineer in London?
Reion is Gas Safe registered (919881), based in South London, and available 24/7. Fixed price after the diagnostic visit — no hidden charges.
