The boiler tried to light at startup but failed to establish a flame, so it has locked out for safety.
Most common on: ecoTEC plus, ecoTEC pro
F.28 is one of the most common Vaillant fault codes. It means the boiler went through its ignition sequence but failed to light — no stable flame was detected during startup — so the control board shut the gas valve and locked out.
It is a safety lockout, not a random error. A single lockout can often be cleared with a reset, but a boiler that locks out on F.28 repeatedly has an underlying fault — most commonly with the gas supply, ignition electrode or gas valve — that needs a Gas Safe engineer.
Make sure a gas hob or another gas appliance lights. If nothing gets gas, the problem is the supply — check the gas cock at the meter is on and that a prepayment meter has credit.
The isolation lever under the boiler should be in line with the pipe (open). If it is turned across the pipe, the gas is shut off.
Press the reset button and wait. If the boiler fires and stays lit, it was a one-off lockout. If it locks out on F.28 again, stop resetting it repeatedly.
In cold weather a blocked or frozen condensate pipe (the white plastic pipe running outside) can trigger a lockout. Thawing it can clear the fault — but if you are unsure, call an engineer.
Repeated F.28 lockouts point to the electrode, gas valve, PCB or gas pressure — all of which need a Gas Safe registered engineer with the right test gear.
Work on the gas side of a boiler must be carried out by a Gas Safe registered engineer. The checks above are limited to safe, non-invasive things a homeowner can look at (pressure, resets, the gas supply) — if in doubt, stop and call an engineer.
TradePlanr turns the callout into paid work — log the job, quote the parts and labour, raise the gas safety record and send the invoice from your phone, even with no signal. Free to start, works in your browser, no download needed.
The boiler tried to light at startup but failed to establish a flame, so it has locked out for safety. F.28 is one of the most common Vaillant fault codes. It means the boiler went through its ignition sequence but failed to light — no stable flame was detected during startup — so the control board shut the gas valve and locked out.
You can try a single reset and the safe checks below, but Vaillant F.28 usually needs a Gas Safe registered engineer. Never work on the gas side of a boiler yourself — it is illegal and dangerous unless you are Gas Safe registered.
A recurring F.28 means the underlying cause has not been fixed — most often no gas reaching the boiler (gas cock off, meter credit run out, or the wider gas supply interrupted). Resetting repeatedly will not solve it; get the boiler diagnosed properly.