Boiler Fault Codes Explained: What Your Boiler Is Actually Telling You (London Guide 2026)

Q: What can I safely check myself when my boiler shows a fault code?

Five things, and they are the same regardless of brand because all five sit outside the boiler casing. First, read the pressure gauge or digital pressure display. Below about 1 bar cold is low, and topping the system up to around 1.2 bar through the filling loop with the boiler switched off and cool is a legitimate homeowner job. Second, find the emergency control valve at the gas meter and confirm the lever sits in line with the pipe, which means the gas is on. Third, try another gas appliance such as the hob: if it will not light either, the problem is the supply rather than the boiler. Fourth, if the property is on a prepayment meter, confirm it is in credit and that any emergency credit has been accepted at the meter itself. Fifth, in cold weather, find the white plastic condensate pipe leaving the property and pour warm water at roughly bath temperature along it, never boiling, because thermal shock can crack the pipe. A frozen condensate pipe is the single most common cause of a January boiler lockout in London and it is one of the few faults a homeowner can genuinely resolve.

Q: Why does the same fault have a different code on every boiler brand?

Because there is no industry standard for fault code labelling, only for the safety behaviour underneath it. Every condensing boiler sold in the UK has to detect the same basic conditions, so they all check water pressure, circulation, flame presence, fan speed and temperature. What each manufacturer calls those checks is entirely their own choice. An ignition failure is EA on a Worcester Greenstar, F.28 on a Vaillant ecoTEC, L2 on an Ideal Logic, E133 on a Baxi or Potterton, and F1 on several Glow-worm ranges. They are all the same complaint: the boiler tried to light and no flame was established. The same applies to low water pressure, which is F.22 on a Vaillant, F1 on an Ideal, E119 on a Baxi and F22 on a Glow-worm. The useful consequence is that once you know which of the four or five underlying faults you have, the homeowner checks are identical across every brand, and only the label and the reset method change.

Q: Is a boiler fault code an emergency?

It depends on the code and on who is in the house. Treat it as a genuine same-night emergency if there is no heating or hot water during cold weather and there is a baby, an elderly person or anyone unwell at home, if the boiler is leaking water, if there is a burning or scorching smell, if the boiler is banging or kettling, or if it keeps locking out and will not stay running. An overheat code that keeps returning belongs in that category too, because an overheat limiter is a last line of defence rather than an inconvenience. Any smell of gas is handled differently and comes first: turn the gas off at the meter's emergency control valve, open windows and doors, avoid electrical switches and naked flames, leave the property if the smell is strong, and ring the National Gas Emergency Service on 0800 111 999. By contrast, a low pressure code on a mild day that clears after a top-up can generally wait for a weekday daytime appointment and the lower call-out rate that comes with it.

Q: My boiler has stopped but shows no fault code. What now?

A boiler with a blank display has usually lost its electrical supply rather than developed an internal fault, so start there. Check the switched fused spur next to the boiler is on, check the consumer unit for a tripped breaker or RCD, and check any timer or programmer is actually calling for heat and has not lost its settings after a power cut. If the display is lit but the boiler simply will not fire, run the same five external checks that apply to any code: pressure gauge, gas meter emergency control valve, another gas appliance, prepayment credit, and the external condensate pipe in cold weather. Older boilers are a separate case. Many units installed before the mid-2000s signal faults with a pattern of flashing neon lamps on the front panel rather than a numeric display, and Baxi and Potterton units in particular do this, so the lamp pattern in your model's user guide is what you need rather than a code table. Whatever you find, note the model name from the front panel and the GC number from the data badge before you ring.

Every boiler fault code names the safety check that failed, not the part that broke. What lockouts and faults mean, the five checks you can safely do yourself, when a reset is fine and when it is dangerous, plus links to the code guides for Worcester, Vaillant, Ideal, Baxi and Glow-worm.

Your boiler has stopped and there is a code on the display. Before you go looking up what your code means, it is worth spending two minutes on what a code is, because the general principle answers most of the questions people actually have and it is the same on every brand ever sold in the UK.

This is the hub page for our boiler fault code guides. It covers what codes are, the difference between a lockout and a fault, the five checks you can safely do yourself whatever your boiler says, when a reset is appropriate and when repeated resetting is genuinely dangerous, and the exact point where the job stops being yours. At the end there are links to the detailed guides for the five brands that make up most of London's install base.

⚠️ Before anything else: if you can smell gas, this is not a boiler repair problem. Turn the gas off at the meter's emergency control valve, open windows and doors, do not touch electrical switches, do not use naked flames, and ring the National Gas Emergency Service on 0800 111 999. It is free and staffed 24 hours. Repairs to the appliance happen after the supply has been made safe.

What a Fault Code Actually Is

Here is the sentence that makes every fault code guide easier to read:

A fault code names the check that failed, not the part that broke.

Every modern condensing boiler runs a sequence of checks before and during firing. It measures the water pressure in the sealed circuit. It confirms water is actually circulating. It watches the fan speed. It looks for a flame at the moment it expects one, and it keeps looking while the burner runs. It monitors flow and return temperatures. If any one of those checks returns an answer the boiler does not trust, it stops and displays the code for that check.

So when a boiler shows a circulation code, it is not saying the pump is dead. It is saying it started the pump and did not observe the water movement it expected. That could be a seized pump, a closed or seized valve, a circuit restricted by magnetite sludge, or simply not enough water in the system for the pump to move. Four quite different repairs, one code.

This is why an online table can tell you what the boiler is complaining about but not what is wrong with it, and why for a lot of codes the honest answer is that the diagnosis itself is engineer work. It is also why the most valuable thing you can write down is the code exactly as displayed, along with your boiler's model name from the front panel and the GC number from the data badge.

💡 Write the code down before you reset. Many boilers clear the code from the display once the fault resets. An engineer who knows you had the same lockout three times overnight will arrive with the right parts on the van, and that is the single cheapest thing you can do to shorten a call-out.

Lockout vs Fault: Why It Matters

Codes fall into two groups, and knowing which one you are looking at tells you how much the boiler has already decided.

A lockout means the boiler has shut itself down and will not restart on its own. Somebody has to press reset. It has judged that a start sequence should not be repeated automatically. Ignition failures are the classic example: the boiler tried to light, got no flame, and will not keep trying indefinitely into a room.
A fault means the boiler has flagged a condition, usually stopped heating, but is watching for that condition to go away and will normally resume by itself once it does. Low water pressure is the everyday example: put the pressure back and most boilers carry on without anyone touching a button.

Some manufacturers make the distinction explicit in the code itself. Ideal is the clearest, using an L prefix for lockouts and an F prefix for faults. Most others do not label it and you can only tell from the behaviour: if the boiler restarts on its own once you have fixed something, it was a fault; if it sits there until you press reset, it was a lockout.

Practically, a fault invites you to fix the underlying condition. A lockout is the boiler telling you it has already made a safety decision. Neither ever means it is acceptable to open the casing.

The Five Checks You Can Safely Do, Whatever the Code Says

These are the same on every brand, because all five sit outside the boiler casing. Run them before you pay for a call-out, and run them in this order.

Read the pressure gauge

Either an analogue gauge on or near the boiler, or a digital pressure reading on the display. Below about 1 bar when the system is cold is low, and most boilers stop somewhere around 0.5 to 0.8 bar. If it is low, top the system up to around 1.2 bar through the filling loop with the boiler switched off and cool, opening the valves slowly and closing them the instant the reading hits 1.2. Our step-by-step repressurising guide covers the valve types you will meet on each brand.

Confirm the gas supply is actually on

Find the emergency control valve at the gas meter. When the lever sits in line with the pipe the gas is on; across the pipe it is off. It is more common than you would think for a valve to have been turned off during other work and never turned back.

Test another gas appliance

Try the gas hob. If the hob will not light either, the problem is the supply rather than the boiler, and that is a very different phone call. If the hob lights normally, gas is reaching the property and the issue is at the boiler.

Check the meter is in credit

On a prepayment meter, a boiler that has run out of credit and been topped back up sometimes needs the emergency credit accepted at the meter itself before gas flows properly again. Worth 30 seconds before you pay a call-out fee.

In cold weather, check the condensate pipe

Find the white plastic pipe leaving the property, usually 22mm or 32mm and often the only white pipe on that wall. After a hard frost it can freeze solid, and a blocked condensate route is the single biggest cause of a January boiler lockout in London, frequently accompanied by a gurgling sound as the boiler tries and fails. Pour warm, never boiling water along it, concentrating on external bends and the exposed end. Boiling water can crack the pipe or its fittings. Our frozen pipes guide for London covers the whole job.

That is the complete list. There is no sixth check, and anything that involves removing a panel, a screw or a cover is not on it.

✓ Free 30-second diagnostic if the pressure keeps dropping: step outside and look at the brass pipe that terminates low on the external wall near the boiler. If it is wet, dripping, or streaked with white limescale, the pressure relief valve has been discharging and the pressure loss almost certainly traces back to a failed expansion vessel. We rank the four causes of pressure loss by how often we actually see them in why your boiler keeps losing pressure.

When a Reset Is Appropriate, and When It Is Dangerous

The reset button is the most misused control on a boiler, so it is worth being precise about it.

A reset without a change is not a repair. That is the whole rule. Resetting after you have topped the pressure up, turned the gas back on, or thawed a frozen condensate pipe is reasonable, because you have changed the conditions and you are asking the boiler to re-evaluate. Pressing reset again five minutes later, having changed nothing, is asking the boiler to run the same sequence it has already judged unsafe.

Two or three attempts, maximum. Manufacturer guidance across the major brands converges on stopping and seeking qualified help once a fault persists through a small number of resets.
A code that clears and returns the same day is a persistent fault. Book the visit rather than resetting through the evening. The pattern is diagnostic information and resetting it away throws that information out.
Never repeatedly reset an overheat code. An overheat limiter is a last line of defence. If a boiler keeps reaching it, that is an emergency call-out rather than an inconvenience.
Turn the boiler off at the switched fused spur if it is banging, kettling loudly or smells of scorching, and ring rather than reset.

The reset method varies. On most boilers it is a dedicated button held for a few seconds; on Vaillant and Glow-worm units it is often a flame symbol; on many older Worcester Greenstars it is built into the central heating temperature dial, which you turn fully to a marked reset position. Your model's user guide shows which. What does not vary is the discipline.

⚠️ Repeated resetting is not a neutral act. A lockout after a failed ignition exists precisely to stop a boiler cycling through unsafe start attempts. Forcing it through that sequence again and again, on an appliance that has already told you something is wrong, is the one homeowner behaviour that turns a cheap fault into a dangerous one.

Same Fault, Different Label: How the Brands Compare

There is no industry standard for fault code labelling, only for the safety behaviour underneath it. Every condensing boiler has to detect the same basic conditions, and what each manufacturer calls those conditions is entirely their own choice. The result is that four or five underlying faults account for most call-outs, wearing a different name on every brand.

What has actually happened	Worcester Bosch	Vaillant	Ideal	Baxi / Potterton	Glow-worm
Not enough water in the sealed circuit	Low pressure warning (varies by model)	F.22	F1	E119	F22
Tried to light, no flame established	EA	F.28	L2	E133	F1 (Betacom, Flexicom, Energy)
Pump ran, no circulation or pressure change detected	A1	F.75	L1	E125 (reported on several ranges)	F75
Overheat safety shutdown	E9	F.20	Check your model's manual	E110	Check your model's manual

Two things follow from that table. The first is reassuring: once you know which of the underlying faults you have, the homeowner checks are identical across every brand, and only the label and the reset method change. The second is a caution: the blank cells are blank on purpose. We have not filled them with plausible-looking codes, because a confident wrong answer about a gas appliance is worse than no answer, and manufacturers reuse the same characters with different meanings across their own ranges.

Find Your Brand's Guide

Each of these goes into the codes we attend most often on that brand in London, with the safe checks and the exact stop point for each one.

Worcester Bosch fault codes: EA, E9 and A1. The largest install base in London. Covers the letter code plus numeric service code pairing that is unique to Greenstar units, the two different reset methods across older and newer models, and C6 fan faults.
Vaillant fault codes: F.75, F.28 and F.22. The ecoTEC range. Covers why F.75 is so often a scaled water pressure sensor rather than a dead pump, and why F.75 and F.22 sometimes appear together.
Ideal fault codes: L2, L1 and F1. Logic and Vogue units, common in London new builds and rental conversions. Covers the L for lockout and F for fault convention, which is the clearest of any brand.
Baxi fault codes: E133, E119 and E110. Also covers Potterton, which shares the same control platforms and code set, and the older units that signal with flashing lamps rather than numbers.
Glow-worm fault codes: F1, F22 and F75. Betacom, Flexicom, Ultimate and Energy. Covers why some Glow-worm codes read directly across from Vaillant and why the low-numbered ones do not.

💡 Not sure which brand you have? The manufacturer name is normally printed on the front panel, and the data badge, usually on the underside of the boiler or behind a flap, carries the model name and a GC number in the format 47-000-00. Between them those two pieces of information identify your boiler precisely, and they are worth photographing once so you are not squinting at a badge in the dark next January.

No Code, or a Code You Cannot Find

Not every stopped boiler shows a code, and not every code appears on a table.

Blank display. The boiler has usually lost its electrical supply rather than developed an internal fault. Check the switched fused spur next to the boiler is on, check the consumer unit for a tripped breaker or RCD, and check the timer or programmer is actually calling for heat and has not lost its settings after a power cut.
Display lit but the boiler will not fire. Run the five checks above. A boiler that is powered and shows no fault but does nothing on a demand is often a controls problem, such as a room thermostat with flat batteries or a programmer that has reverted to an off schedule.
Flashing lamps instead of a display. Many boilers installed before the mid-2000s signal faults with a pattern of neon lamps on the front panel rather than a numeric code, and older Baxi and Potterton units in particular do this. The lamp pattern in your model's user guide is what you need, not a code table.
A code that is not on any list you can find. Do not guess at it, and be wary of the first result that offers a confident meaning. Manufacturers reuse characters across ranges. Note it exactly as displayed, note the model name and the GC number, and quote all three when you ring.

One symptom worth calling out separately: if you have heating but no hot water, or hot water but no heating, and there is no fault code at all, that pattern points at the diverter valve rather than anything the boiler will tell you about. We cover it in no hot water but heating works.

Where You Must Stop: The Gas Safe Line

This is the part that matters more than any code on this page. Under the Gas Safety (Installation and Use) Regulations 1998, work on a gas appliance in the UK may only be carried out by an engineer on the Gas Safe Register. Removing the casing of a sealed-system gas boiler counts as work on the appliance. It is a criminal offence for anyone else to do it, it invalidates the boiler warranty and often the home insurance, and it puts you alongside live mains electrics, a pressurised water circuit and a gas carrying component simultaneously. It also breaks the room-sealed integrity of the appliance, which is what keeps combustion products in the flue rather than in your home.

So the boundary is simple and it is physical: outside the casing is yours, inside the casing is not.

💡 Outside the casing, and yours to do: reading the pressure gauge or digital pressure display, using the external filling loop, checking the gas meter's emergency control valve and other gas appliances, confirming prepayment credit, thawing an external condensate pipe with warm water, checking the outside pressure relief discharge pipe for drips, checking the room thermostat and radiator valves, checking the fused spur and consumer unit, using the reset control, and writing the fault code and model name down.

⚠️ Inside the casing, and never yours to do: the pump, the water pressure sensor, the gas valve, the ignition and flame sensing electrodes, the fan, the overheat limiter, the expansion vessel, the pressure relief valve, the diverter valve, the heat exchanger, the PCB and the flue. There is no exception to this for "just having a look", and there is no circumstance in which bypassing or defeating a safety device is acceptable.

When an engineer does attend, ask to see their Gas Safe ID card and check the registration number at gassaferegister.co.uk. The card shows which appliance types they are qualified to work on, and the number on the card should match the one on your invoice.

What These Repairs Typically Cost in London (2026)

Indicative London pricing, and it is brand-agnostic: the diagnosis time and the labour are much the same whichever badge is on the front, with parts prices the main variable. The call-out covers travel and the first 45 minutes on site including diagnosis, and any parts are quoted before fitting rather than after.

Job	Typical London cost (2026)	Notes
Call-out + diagnosis (weekday daytime)	£60	Includes first 45 minutes on site
Call-out + diagnosis (out-of-hours / weekend)	£90	Same 45-minute window
Repressurise, reset or minor fix (no parts)	£60–£120	Often completed inside the call-out window
Frozen condensate re-route + lag	£90–£160	Stops the same lockout recurring every cold snap
Expansion vessel / pressure relief valve	£120–£280	The usual cause behind repeated pressure loss
Diverter valve replacement	£150–£320	Common combi hot-water fault
Pump replacement	£180–£380	Part and labour, one of the circulation-fault outcomes
Fan replacement	£200–£420	Part price varies by model
PCB (circuit board) replacement	£350–£650	Often the repair versus replace tipping point
Primary heat exchanger	£450–£750+	Scale-related, and a frequent overheat outcome; may favour replacement

Two things are worth knowing before you book. First, the call-out is not dead money: on most fault code jobs the diagnosis and a minor fix both happen inside that first window, so plenty of visits finish at the £60 to £120 mark. Second, if the diagnosis lands on a PCB or a heat exchanger on a boiler that is already 12 to 15 years old, pause and price a replacement before committing. Our emergency boiler repair guide for London sets out the full repair versus replace maths.

Landlords: A Fault Code Is a Repair Clock Starting

If you let the property, a locked-out boiler is a legal matter as well as a practical one. Under Section 11 of the Landlord and Tenant Act 1985 you must keep the installations for space heating and hot water in repair and proper working order. Under the Homes (Fitness for Human Habitation) Act 2018, a home left without heating or hot water, particularly in winter or where a child, an elderly or a vulnerable tenant lives, can be judged unfit for human habitation.

There is no single statutory deadline, but the accepted benchmark is to act within roughly 24 hours for a total loss of heating or hot water, and same day where a vulnerable person is affected. Three practical points: ask the tenant to photograph the display showing the code and the model name from the front panel, which speeds up the dispatch and gives you a dated record; never coach a tenant into opening the boiler, because the Gas Safe restriction applies to them exactly as it applies to you; and keep a note of which brand and model is in each property, because it is the difference between an engineer arriving with the part and arriving to diagnose. Separately, the annual Gas Safety Record (CP12) from a Gas Safe engineer is required for every gas appliance in the property regardless of whether anything has broken.

Frequently Asked Questions

What does a boiler fault code actually mean?

A fault code names the safety check that failed, not the part that broke. Every condensing boiler runs a sequence of checks on water pressure, circulation, fan speed, temperature and flame presence, and when one of those checks returns an answer it does not trust, it stops and shows the code for that check. So a circulation code does not mean the pump is dead: it means the boiler started the pump and did not observe the water movement it expected, which could be a seized pump, a closed valve, a sludged circuit, or too little water in the system. That is why an online table can tell you what the boiler is complaining about but not what is wrong with it.

What is the difference between a boiler lockout and a boiler fault?

A lockout means the boiler has shut itself down and will not restart until somebody presses reset, because it has judged that a start sequence should not be repeated automatically. Ignition failures are the classic example. A fault means the boiler has flagged a condition and usually stopped heating, but is watching for it to clear and will normally resume by itself once it does, and low water pressure is the everyday example. Ideal makes the distinction explicit with an L prefix for lockouts and an F prefix for faults; most other brands do not label it and you can only tell from the behaviour. Neither ever means it is acceptable to open the casing.

What can I safely check myself when my boiler shows a fault code?

Five things, identical on every brand because all five are outside the casing. Read the pressure gauge and top up to around 1.2 bar through the filling loop if it is below about 1 bar cold, with the boiler off and cool. Confirm the emergency control valve at the gas meter is in line with the pipe, which means gas is on. Try another gas appliance such as the hob: if it will not light either, the problem is the supply. If you are on a prepayment meter, confirm it is in credit and any emergency credit has been accepted at the meter. In cold weather, pour warm water at roughly bath temperature, never boiling, along the external white condensate pipe. That is the complete list.

How many times can I reset my boiler?

Two or three attempts at most, and only after you have made an actual change such as topping the pressure back up, turning the gas back on or thawing a frozen condensate pipe. A reset without a change is not a repair. A lockout is a deliberate safety response, and pressing reset repeatedly on a fault you have not addressed asks the boiler to run the same sequence it has already judged unsafe. It also destroys useful evidence: a code that clears and returns three times in an evening tells an engineer something a single reset does not. Note the code before you reset, because many boilers clear it from the display afterwards.

Why does the same fault have a different code on every boiler brand?

Because there is no industry standard for the labels, only for the safety behaviour underneath them. Every condensing boiler has to detect the same conditions, and what each manufacturer calls them is their own choice. An ignition failure is EA on a Worcester Greenstar, F.28 on a Vaillant ecoTEC, L2 on an Ideal Logic, E133 on a Baxi or Potterton and F1 on several Glow-worm ranges. Low water pressure is F.22 on a Vaillant, F1 on an Ideal, E119 on a Baxi and F22 on a Glow-worm. The useful consequence is that once you know which underlying fault you have, the homeowner checks are the same everywhere and only the label and the reset method change.

Is a boiler fault code an emergency?

It depends on the code and who is in the house. Treat it as a same-night emergency if there is no heating or hot water in cold weather with a baby, an elderly person or anyone unwell at home, if the boiler is leaking water, if there is a burning smell, if it is banging or kettling, or if it keeps locking out and will not stay running. A repeating overheat code belongs in that category. Any smell of gas comes first and is handled differently: turn the gas off at the meter, ventilate, avoid switches and flames, and ring 0800 111 999. A low pressure code on a mild day that clears after a top-up can usually wait for a weekday daytime slot at the lower call-out rate.

Can I open my boiler to look at the fault myself?

No. Under the Gas Safety (Installation and Use) Regulations 1998, work on a gas appliance is restricted to Gas Safe registered engineers, and removing the casing of a sealed-system gas boiler counts as work on the appliance. It is illegal, it invalidates the warranty and often the home insurance, and it puts you next to live electrics, a pressurised water circuit and a gas carrying component at once. It also breaks the room-sealed integrity that keeps combustion products in the flue. Outside the casing is yours, inside the casing is not, and everything genuinely useful to a homeowner is on the outside.

My boiler has stopped but shows no fault code. What now?

A blank display usually means the boiler has lost its electrical supply rather than developed an internal fault, so check the switched fused spur next to the boiler, the consumer unit for a tripped breaker or RCD, and whether the timer or programmer is actually calling for heat after a power cut. If the display is lit but the boiler will not fire, run the same five external checks that apply to any code. Older boilers are a separate case: many units installed before the mid-2000s, Baxi and Potterton among them, signal faults with a pattern of flashing neon lamps rather than a numeric display, so the lamp pattern in your model's user guide is what you need.

The Bottom Line

A fault code is the boiler telling you which of its safety checks failed. It is genuinely useful information, but it is a starting point rather than a diagnosis, and the gap between the two is exactly the gap that a Gas Safe engineer with a meter is there to close.

What you can do is consistent and it is short: read the pressure and top up if it is low, confirm the gas is on and another appliance lights, confirm a prepayment meter is in credit, and thaw the external condensate pipe with warm water in cold weather. Then reset once, twice at the very most, and stop. Note the code exactly as displayed along with your boiler's model name, and go to the guide for your brand for the specifics.

And if you smell gas at any point, none of the above applies. Turn the gas off at the meter, ventilate, avoid switches and flames, and ring 0800 111 999 before anything else.

Emergency Repairs London provides same-day and 24/7 emergency boiler repair with Gas Safe registered engineers across all 32 London boroughs, covering Worcester Bosch, Vaillant, Ideal, Baxi, Potterton and Glow-worm diagnostics and repairs. Fixed £60 daytime call-out, quoted before work starts. Lines are open 24/7 on 0207 046 1363.