Boiler not firing up
The display is lit, the pump may even be running, and the burner never lights. Usually the boiler has been told not to fire, or it tried and gave up.
What it usually means
There is a fixed order of events between something asking for heat and the burner lighting, and a boiler that will not fire has stopped at one of the steps. Knowing the order is what turns this from a mystery into a short list.
First something has to ask. A programmer, a room thermostat, a cylinder thermostat or the flow sensor that notices you have opened a hot tap — one of those has to close a circuit. Second, the appliance checks its own conditions: enough water pressure, water actually moving, the fan reaching speed, a clear flue and somewhere for the condensate to go. Only then does it open the gas valve, spark, and look for a flame it can prove.
That structure is why so many of these calls end without a part being fitted. A thermostat with flat batteries, a programmer that quietly reset itself to off after a power cut, a smart control that has lost the house wi-fi, or every radiator valve in the house satisfied on a mild evening all produce a boiler that is idle, healthy and doing as it was told. None of them shows a fault code, because there is no fault.
The condition checks are the next band, and one of them is seasonal. A condensing boiler has to dispose of the acidic water it makes, and if the plastic pipe carrying it away runs outside, a hard frost freezes it. The boiler then shuts down deliberately rather than let that water back up inside itself — and it usually says so with a code, in the depths of winter, on the coldest morning of the year.
Past that point you are into ignition and flame proving, where the boiler has genuinely tried. Most makes allow three attempts and then lock out on the gas valve. The causes there run from things you can check in a minute — whether the gas is on at all, whether a prepayment meter has credit, whether the hob lights — to an electrode, a fan or a gas valve, and everything in that second group is Gas Safe work by law rather than by preference.
How to narrow it down
Work through these in order.
Is the display lit at all?
If there is nothing on it, this is not an ignition problem — the appliance has no power. Check the switch on the fused spur beside it, then the consumer unit for a tripped breaker or residual current device, remembering that a boiler often shares a circuit with sockets and can be knocked out by a completely unrelated appliance failing.
Does it fire when you run a hot tap?
This is the fastest way to separate the burner from the controls. A boiler that lights instantly for hot water and never for the radiators has a working burner and a problem on the heating side — the programmer, the room thermostat, the wiring centre or a motorised valve — and no amount of resetting will change that.
Is there a code on the display, and does it come straight back after a reset?
Write down exactly what it shows, including any letters and any three-digit suffix, and look it up rather than guessing. A code that clears and stays clear for days is worth noting. One that returns within a minute of every reset is telling you the condition it is complaining about has not changed.
Has it been below freezing?
Find the plastic pipe leaving the boiler and going outside, usually white or grey and about the width of a waste pipe. If it is frozen the boiler will not run until it is thawed, and the fix is warm water poured along the outside of the pipe rather than boiling water, which can split it.
Do your other gas appliances work?
If the hob will not light either, there is nothing wrong with the boiler and no gas at the property. Check that any prepayment meter has credit, and that the emergency control valve handle at the meter is in line with the pipe rather than across it, before you book anybody.
Does it light and then go out again after a few seconds?
That is a different fault from one that never lights, and it is worth saying so on the phone. Establishing a flame and then losing it points at the gas supply under load, the flame sensing circuit or a flue problem, and the manufacturers give it its own separate code for exactly that reason.
How many times have you reset it?
Stop at two. Manufacturers limit resets deliberately — Ideal boilers show LC after five in a quarter of an hour and then refuse — because repeatedly commanding a gas valve to open into a chamber that has already failed to light is not a diagnostic technique. A lockout that returns twice needs an engineer, not a third go.
Codes commonly shown with this
- F.28VaillantThe 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 — call an engineer
- F.54VaillantA gas supply fault, usually shown alongside F.28 or F.29. It means gas is reaching the appliance at too low a pressure.Get it checked
- LCIdealYou have reset the appliance five times in fifteen minutes. This is not a fault in itself — it is the boiler asking you to stop.Usually simple to fix
- F4ViessmannNo flame signal was detected during burner start on any two-character Vitodens, so the burner has locked out. Reset once; if it returns, call a Gas Safe engineer.Stop — call an engineer
- E01AlphaThe appliance tried to light and could not confirm a flame, so it has locked out. In winter a frozen condensate pipe is the most likely reason.Get it checked
- A01VokèraThe burner failed to light and the appliance has locked out. Check the gas supply and, in cold weather, the condensate pipe before calling anyone.Get it checked
When it is urgent
Take care
What it costs
Typical repair cost
£0–£450
Typical UK prices including parts and labour, updated September 2026. Your quote may differ.
Get an engineer to look at this
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.
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.