Boiler Fault Codes, Flashing Lights and Lockouts Explained

When your boiler stops working, it almost always tells you why. Modern UK boilers display a fault code, a flashing letter and number, or a sequence of lights on the front panel, and each code points to a specific problem: low water pressure, a failed ignition, a blocked condensate pipe or a sensor fault. The trick is knowing which codes you can act on yourself and which mean the boiler needs a Gas Safe registered engineer.

The first move with almost any code is the same: reset the boiler once, and if the code comes straight back, stop and diagnose rather than resetting again. Ideal Heating, one of the UK's most common boiler makers, warns that resetting too many times in 15 minutes triggers its own lockout code, L5¹. Two checks solve a large share of faults without any tools: the system pressure gauge should read 1 to 1.5 bar¹, and if you are on a pay-as-you-go gas meter, there should be credit on it¹.

One thing comes before every fault code: if you can smell gas, that is not a boiler fault to diagnose, it is an emergency. Open a window if you can, leave the property immediately and call the National Gas Emergency Service on 0800 111 999². Do not smoke or use naked flames until you are sure there is no leaking gas².

If you smell gas, leave the property and ring the emergency line first

Before any fault code matters, deal with gas. If you smell gas inside or outside your home, open doors and windows, leave the area and call the National Gas Emergency Line on 0800 111 999². This is a free, 24-hour national number, and you should ring it whether the smell is inside or outside the property⁵. Fire and rescue services across the UK give the same advice: Scottish Fire and Rescue Service directs anyone who smells gas and suspects a leak to ring the National Gas Emergency Service on 0800 111 999⁶, and Cornwall Council lists the same number for gas leaks⁷.

Do not use electric switches, do not smoke, and extinguish any naked flames. The Royal Voluntary Service's winter guidance adds that you should turn off the gas at the meter unless the meter is in the cellar, in which case do not go down into it, because gas can collect there⁸. Once the leak has been stopped, any work on appliances such as cookers, boilers or fires has to be carried out by a Gas Safe registered engineer⁹.

In Northern Ireland the number is different: if you smell gas, call the NI Gas Emergency Service on 0800 002 001³. Tenants in rented homes are given the same 0800 111 999 number to call if they smell gas or suspect a carbon monoxide leak¹⁰. The Health and Safety Executive also says to call the National Gas Emergency Service on 0800 111 999 if you think your appliance is spilling carbon monoxide¹¹. Our page on the National Gas Emergency Service explains what happens when you ring.

Reset the boiler once before anything else

Most fault codes can be cleared the same way, and the maker's own guidance is consistent about it: reset once, and only once, before you start investigating. On Ideal boilers, the advice for several codes is simply to reset the boiler¹²; for others, such as L5, the maker's instruction is to turn the power to the boiler off and on again¹². If a single reset clears the code and the boiler runs normally afterwards, the fault was probably a one-off, such as a brief interruption in the gas or electricity supply.

If the code comes back immediately, do not keep resetting. Ideal Heating's own fault code list shows why: code L5 exists specifically to tell you the boiler has been reset too many times in 15 minutes¹. The boiler is protecting itself. Instead, use the code to narrow down the cause:

1. Note the exact code or light sequence, including which characters are flashing.
2. Reset once, following the maker's instruction for that code.
3. If it returns, check the two most common household causes: the pressure gauge (it should read 1 to 1.5 bar) and, on a prepayment meter, that you have credit¹.
4. If neither fixes it, match the code to the tables below and follow the maker's advice for it.

For ignition lockouts specifically, Ideal's troubleshooting manual for the Logic system range sets out the first steps in order: check the boiler's condensate pipe for blockages, then check whether other gas appliances are working¹³. A frozen or blocked condensate pipe is one of the most common causes of winter lockouts, and our page on frozen condensate pipes covers how to spot and safely thaw one.

Lockouts: what a flashing flame or L code means

A lockout is the boiler refusing to fire until the fault is cleared. On Ideal boilers, lockout codes begin with L. The most common is L2, which Ideal's troubleshooting manual defines plainly: "L2, Ignition Lockout"¹³. The support articles describe the same code as Flame Loss or Ignition Lockout, shown as a flashing L and 2¹. In plain terms, the boiler tried to light, failed, and shut down safely rather than pumping unburned gas into the burner.

Ignition lockouts have a short list of usual causes, and Ideal's own guidance walks through them. If your gas meter is pay-as-you-go, check there is credit available¹. If other gas appliances are also not working, or you have no other gas appliances to check, look at whether the gas supply is on at the meter¹³. Check the condensate pipe for blockages, which in cold weather usually means ice¹⁴. If the boiler is making a gurgling or gargling noise whilst trying to fire, contact your installer, as there may be a blockage¹. If none of that fixes it, call Ideal Heating quoting Error Code L2¹.

Other L codes on Ideal boilers point to different faults, and the maker's advice differs code by code:

The pattern to notice: L1, L2, L3 and L5 have household checks attached to them. L4 and L6 do not, and the maker sends you straight to a professional. That distinction, more than the code itself, is what decides whether you can act.

Pressure faults: keep the system at 1 to 1.5 bar

Low water pressure is the fault most households can fix completely on their own, and it is the one to check whenever the boiler shows a pressure-related code or the heating runs poorly. Ideal Heating states the correct range repeatedly across its guidance: boiler water pressure should be at 1 to 1.5 bar¹. On Ideal boilers, code F1 (flashing F and 1) means Low Pressure: the water pressure is too low in the system and will require topping up to between 1 and 1.5 bar¹.

The maker's instruction for F1 is to check the system water pressure is between 1 and 1.5 bar on the gauge and, if that is the issue, re-pressurise the system as outlined in the user manual¹³. Our step-by-step page on how to repressurise a combi boiler walks through the filling loop, and the fuller background is on boiler pressure.

Pressure and air in the system are linked, and Ideal's advice for several codes combines the two. For code L3, the instruction is to turn the boiler off, bleed the air out of the radiators, ensure the boiler pressure is still at 1 to 1.5 bar, and turn the boiler back on¹. Radiators need bleeding when they are colder at the top than the bottom, because that means excess air is trapped¹⁶. But bleeding should not become a routine: if it is needed regularly, it is likely hiding a wider issue, and a Gas Safe registered engineer should check the system¹⁶. Our page on how to bleed a radiator gives the full method.

Ideal boiler codes: F1 to F7 and the other display messages

Ideal Heating publishes its fault code meanings in its support articles, and the F series covers sensor, fan and supply faults. Several of these have no household fix, and the maker says so directly.

Two of these deserve a note. F2 is the code where Ideal's warranty guidance is explicit: if you can see no reason the boiler shouldn't be working, get in touch with Ideal Heating if the boiler is under warranty, or a Gas Safe registered engineer if it is not¹³. F7 is unusual because the maker sends you to an electrician rather than a heating engineer: the voltage to the boiler may be too low, which points to the property's electrics, not the appliance¹.

Ideal's displays also carry codes outside the main series. Flashing C and 0 is a BCC Activation Fault¹. Code LC:5 means the boiler has been reset too many times in 15 minutes, and the fix is to turn the power off and on again¹. Code DU's instruction is to rotate all knobs fully clockwise, turn the boiler off, turn it back on and then press restart¹⁷. Code FA means contact the installer¹⁷. If you own an Ideal Logic model, our product pages for the Logic Combi2, Logic Max Combi2 and Logic+ Heat2 H go into model-specific detail.

Worcester Bosch display symbols that are not faults

Not everything flashing on a boiler front panel is a fault. Worcester Bosch boilers display several symbols that look alarming but are normal operating modes, and the maker's support article is reassuring about each of them. The one that catches most people out is siphon fill mode: the boiler runs at a low burn for about 15 minutes to fill the condense trap. Worcester Bosch's own words are worth having to hand:

The same 15-minute window applies to other non-fault symbols. Maximum mode and minimum mode should each last no longer than 15 minutes⁴. Air purge mode also lasts about 15 minutes, then the boiler returns to normal operation⁴. For maximum and minimum mode, the maker's advice is identical: leave the boiler alone for 15 minutes, and if the symbol is still showing after 15 minutes, ring Worcester Bosch on 0330 123 9559⁴.

The Key Lock symbol is the other one that generates needless call-outs: it means the front panel has been locked, and it applies to the New Greenstar 24/28 only⁴. It is a child lock, not a breakdown. If your Worcester Bosch boiler runs on oil rather than gas, our page on oil boiler breakdowns covers the equivalent lockout and reset behaviour.

Baxi neon lights and Glow-worm fascia error messages

Not every boiler uses a two-character display. Baxi and Glow-worm boilers commonly signal faults with neon lights on the fascia and, on some models, error messages on a small panel. Glow-worm uses an F-numbered code series on its hxi and Energy ranges. The meanings are published in British Gas's guide to Glow-worm boilers:

The specific F-code meanings for Glow-worm's hxi and Energy ranges are not reproduced here. Check the documentation that came with your boiler, or the maker's support pages for your exact model, before acting on any code.

The household logic is the same as for Ideal codes. F22, Low water pressure, is the one you can act on yourself: check the gauge and top the system up, exactly as with Ideal's F1. F4, Ignition fault, sits in the same family as ignition lockout on other makes, so the same first checks apply: prepayment credit, the gas supply at the meter, and the condensate pipe. F10, F16 and F26 describe sensor, flame-detection and circulation faults inside the boiler, and none of them has a household fix: they need a Gas Safe registered engineer.

If your boiler signals with lights rather than codes, the same discipline applies. Note exactly which neons are lit, flashing or off before you reset, because that pattern is the machine's fault code, and an engineer will ask for it. Write it down before you touch anything.

Prepayment meter error codes

If your gas supply is on a pay-as-you-go meter, the meter itself can be the reason the boiler will not fire, and the meter shows its own error codes. Phoenix Energy, the gas supplier in Northern Ireland, publishes the meanings for its Pay as You Go meters:

This connects directly to boiler fault codes: Ideal's guidance for ignition lockout says to check there is credit available if your gas meter is pay-as-you-go¹, and its troubleshooting manual says that if other gas appliances are also not working, check whether the gas supply is on at the meter, and with a prepayment meter, check whether it has credit¹³. Our pages on prepayment meters and on a prepayment meter that has credit but no supply cover the fuller steps.

Faults that need a Gas Safe registered engineer, and never opening the casing

The dividing line is simple: anything that involves opening the boiler, or working on the gas side of it, is a job for a Gas Safe registered engineer. The Royal Voluntary Service's winter guidance is direct: for gas appliance and boiler problems, call a Gas Safe registered engineer⁸. Gas Safe Register's own boiler guide adds the routine that prevents many of these faults in the first place: get your gas boiler and appliances serviced annually by a Gas Safe registered engineer¹⁶. Cheshire Fire and Rescue Service gives a free helpline for finding one: 0800 408 5500¹⁹.

When the engineer arrives, check their credentials. Warwickshire Council's home fire safety guidance says to check Gas Safe ID cards, which should display their photo and details of their gas registration²⁰. Our page on finding a Gas Safe registered engineer and checking their card gives the full method, and boiler servicing explains what the annual visit should cover.

Never open the boiler casing yourself, whatever the code. The components behind it carry mains electricity, gas and, on a running boiler, hot water under pressure. Any work on the boiler itself has to be carried out by a Gas Safe registered engineer⁸. A householder's access stops at the front panel.

Some warning signs go beyond fault codes and mean the boiler is unsafe rather than merely faulty. Cheshire Fire and Rescue Service's Safe and Well booklet lists them: turn the boiler off and call a Gas Safe engineer for strange noises, leaks, a yellow flame, unusual smells, the boiler turning itself on and off, low pressure or no hot water, or soot and staining around the boiler²¹. A healthy gas flame should always be blue²¹.

Carbon monoxide has no smell, which is why the flame colour and staining matter so much. The Health and Safety Executive says to call the National Gas Emergency Service on 0800 111 999 if you think your appliance is spilling carbon monoxide¹¹, and Manchester Fire and Rescue Service advises getting a Gas Safe registered engineer to inspect your gas appliances and flues to check whether there is a dangerous problem²³. The government's own winter advice lists yellow instead of blue flames from gas appliances as a sign of a possible fault, although fuel-effect, living-flame and decorative-flame gas fires are designed to look like solid fuel flames²⁴. If anyone in the household feels unwell with these signs, treat it as a medical emergency. Our page on unsafe appliances covers what happens next.

Where to get help

Help with a boiler fault comes in three tiers, and picking the right one first saves time. For a fault code with a household fix, the maker's own support guidance is the place to start, and this page sets out what Ideal, Worcester Bosch and Glow-worm publish. If the fix does not work, call the maker quoting the error code shown on the display¹. And where the maker says a code needs an engineer, it is telling you the fault cannot be fixed from outside the casing, and a visit will be required¹.

If you rent, our page on broken boilers in rented homes sets out what your landlord must do. If a long outage or a breakdown in cold weather is the underlying cause, the section guide to gas and heating failures and the page on boiler breakdowns in cold weather pull together the wider checks, and extra support from networks and suppliers explains what help exists for households that need it.