Boilers fault codes by brand
Every domestic gas boiler sold in the UK since 2005, and every oil boiler since 2007, is a condensing boiler with a control board that reports faults as a code. Find your manufacturer below, then your code, and you will get the same five answers every time: what it means, whether it is safe, what causes it, what you can try, and what a repair costs.
Brands
Pick your manufacturer to see every code we cover.
Most looked-up boiler codes
Across every brand in this category.
- 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
- EA 227Worcester BoschThe boiler tried to light and never detected a flame, so it has shut the gas off and locked out. This is the most common Worcester fault code.Stop — call an engineer
- F.22VaillantThe 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.Get it checked
- F.75VaillantThe 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.Get it checked
- F1IdealSystem pressure has dropped below the minimum the boiler will run at. Topping it back up to 1.2 bar usually clears it straight away.Get it checked
- 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
- 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
- CE 207Worcester BoschSystem water pressure has dropped too low for the boiler to run safely. Topping the system back up to 1.2 bar usually clears it in five minutes.Get it checked
- 501AristonNo flame was detected during ignition and the appliance has locked out. Check the gas supply and the condensate pipe before booking anyone.Get it checked
- E133BaxiBaxi's ignition lockout: "the gas supply has been interrupted, ignition has failed or the flame has not been detected". Check the gas is on, reset once. If it returns, stop.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
- E10AlphaAlpha describe E10 as the primary pressure switch operating. Refilling to their green band usually clears it, but the switch itself is on their own list of causes.Get it checked
- E118BaxiLow water pressure on the digital-display Baxis: 600, 800, Duo-tec 2, Compact, Assure, Megaflo. Defined by Baxi as primary pressure under 0.5 bar. Repressurise and it clears itself.Get it checked
- L2IdealThe boiler failed to detect a flame during ignition and has locked out on the gas valve. Do not keep resetting it.Stop — call an engineer
- 108AristonThe heating circuit does not contain enough water to run safely. Refill through the filling loop to about 1.2 bar and the code clears.Usually simple to fix
- E119PottertonThe sealed system on a rotary-switch Potterton — Promax, Titanium, Gold, Heatmax — has dropped below half a bar. The window shows E1 and then 19. Fill to 1–1.5 bar and it clears itself.Get it checked
- F.28Glow-wormThree ignition attempts have failed and the boiler has locked out. Glow-worm's guides let you check the gas supply and the condensate pipe and reset three times before calling anyone.Get it checked
- F.74ViessmannSystem pressure too low on a 2020-on Vitodens with the numeric display. The burner and pump stop until the pressure is back. Viessmann tell the householder to top up below 1.0 bar.Get it checked
- A04VokèraSystem water pressure is below the minimum the appliance will run at. Topping the system back up to 1.2 bar normally clears it straight away.Usually simple to fix
- E119BaxiLow water pressure on the selector-switch Baxis — Duo-tec Combi HE, Platinum, Megaflo System HE A. Shown as E1 then 19. Repressurise; no reset is needed.Get it checked
Symptoms
No code on the display? Start from what it is doing.
Boiler keeps losing pressure
A sealed heating system does not use water, so a gauge that keeps falling means water is escaping somewhere. Here is how to work out where.Boiler kettling
The sound of a kettle coming to the boil inside the boiler is water flashing to steam against the heat exchanger, and it is doing damage while you listen to it.Boiler making a banging or kettling noise
A boiler that bangs, rumbles or sounds like a kettle boiling is telling you that water is not moving through it properly. It is worth acting on rather than living with.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.Boiler pressure too high
Above about two and a half bar cold, either something is adding water or something has stopped absorbing it. One test tells you which, and only one of them costs money.No heating and no hot water at all
Before assuming the boiler has failed, there are four things worth checking that account for a large share of these call-outs and cost nothing to rule out.No hot water
What has failed depends entirely on how your house makes hot water, and there are three arrangements in use. Identifying yours takes one look in the airing cupboard.No hot water but the heating still works
When heating works and hot water does not, the fault is almost always in the part of the boiler that switches between the two rather than in the boiler itself.Radiators not getting hot
Which radiators, and which part of each one, is the whole diagnosis. Cold at the top and cold at the bottom are different faults with different answers.Vaillant boiler with no hot water
Read the display before anything else. Vaillant separate status messages from faults, and an ecoTEC that appears to have stopped is often telling you it is deliberately waiting.Water leaking from the boiler
A wet boiler is not one fault but three, and you can tell them apart before anyone opens the case. What the water is decides both the urgency and the bill.Worcester boiler with no hot water
On a Greenstar there are two settings and one dial that produce this complaint without anything being broken, and they are worth checking before the diverter valve gets blamed.
How boilers fault codes work
A boiler fault code is not a diagnosis. It is the control board telling you which of its safety checks failed, and the same code can have four or five different underlying causes. That is why "F1 means low pressure" is true but not useful on its own — the question that matters is why the pressure fell.
Codes fall into a handful of families that repeat across every manufacturer. Pressure faults mean the sealed system has lost water. Ignition faults mean the boiler tried to light and could not confirm a flame. Overheat faults almost always mean water is not circulating rather than that the burner is running too hard. Fan and flue faults mean the boiler cannot verify that combustion products are being cleared. Once you know which family you are in, the diagnosis follows.
Two rules are worth knowing before you touch anything. First, a lockout is a safety device doing its job, and resetting it repeatedly to force the boiler to run is how expensive components get destroyed. Two resets is the sensible limit for any code. Second, gas work is legally restricted to Gas Safe registered engineers in the United Kingdom; an oil-fired boiler is not covered by that law, and the recognised standard for it is an OFTEC registered technician, which is what its pages here say. Checking a pressure gauge, opening a filling loop, or thawing a condensate pipe are all things you can do. Removing the case is not.
Pressure
The sealed system has lost water and the boiler will not fire without it.
Ignition and gas
The boiler tried to light and could not confirm a flame.
Overheat and circulation
Heat is being produced faster than the water can carry it away.
Fan and flue
The boiler cannot verify that combustion products are being cleared.
Sensors and electronics
A sensor or the control board is reporting a value that cannot be right.
Guides
Boiler fault codes explained: what they actually tell you
Codes look like diagnoses and are not. Understanding which family a code belongs to turns a meaningless number into a genuinely useful starting point.How to repressurise a boiler
The five-minute job that resolves more no-heating calls than any other. Here is how to do it properly, and how to tell when topping up is the wrong answer.What a Gas Safe engineer actually checks in a boiler service
Most people pay for this once a year and stand in another room while it happens. Here is what should be going on, which test cannot be faked, and how to tell a real service from twenty minutes and an invoice.What to do when your condensate pipe freezes
The single most common winter boiler fault, and one of the few you can genuinely fix yourself in fifteen minutes with a kettle and some patience.