Boiler fault codes explained: what the code on your display is really telling you

A fault code is not a diagnosis. It is the boiler telling you which of its own safety checks failed. That is a much narrower piece of information than most people assume, and confusing the two is why so many households pay for a visit that fits the wrong part, or replace a boiler that did not need replacing.

This page is not another list of codes. Every manufacturer already publishes one, and theirs is the only list worth trusting for your appliance. This is the thing nobody writes down: how to read any code, what it can and cannot tell you, what is genuinely safe for you to do, and how to work out whether the code in front of you is a repair or the start of a replacement conversation.

Not everything on the display is a fault

Start here, because a good share of the panic is unnecessary. Modern boilers show status information on the same screen they show faults on, and the two look alike to anyone who has not read the manual.

Worcester Bosch, for example, publish a table of operational status codes with the heading that these are not faults. A Greenstar showing 200 is heating the house. 203 is waiting for the system to cool. 270 is powering up. 283 is preparing to light the burner. 357 is an air purge program that runs for about a hundred seconds. 358 is the three way valve exercising itself so it does not seize. On the same range, a boiler that has just been switched back on runs at low output for around fifteen minutes to fill the condensate trap, which is a safety feature and not a fault at all.

There are service reminders too. Worcester's SE code is not a fault: it appears after the boiler has run for 2324 hours, roughly a year of use, and it means a service is due. And the most human entry in any manufacturer's table belongs to Worcester as well: Fd means the reset button was pressed by mistake.

Source: Worcester Bosch, Greenstar i ErP user instructions 6 720 806 946; Worcester Bosch, what do the error symbols mean on my boiler

Why the same code means different things on different boilers

There is no industry standard. Each manufacturer numbers faults against its own control board, so a code is only meaningful next to the badge on the case.

F1 on an Ideal Logic is low water pressure. On a Vaillant, low water in the heating system is F.22. Worcester do not use plain F numbers for most of it at all: they use a letter and a number, so C6 is fan speed too low, E9 is the safety temperature limiter in the central heating flow having tripped, and EA is flame not detected. Three manufacturers, three schemes, no crossover.

The practical consequence: a general answer you find for your code is worth very little unless it was written about your brand. Get the make off the front of the case and the model off the data plate, then use the manufacturer's own list. Everything else, including this page, is context around that.

Source: Ideal Heating, Logic Combi troubleshooting guide; Vaillant, boiler fault codes list; Worcester Bosch, Greenstar CDi Classic user instructions 6 720 812 179

The code names the failed check, not the failed part

This is the point that saves people money. A code tells you which sensor or safety device said no. It does not tell you why.

An overheat code is the clearest example. The boiler is saying its flow got too hot, and on an older system that is usually not a broken thermostat, it is water not moving properly: a sludged heat exchanger, a tired pump, air in the circuit. A flame not detected code is another. It reads like a boiler fault, and it is very often no gas arriving, a frozen condensate pipe in January, or a restricted flue. A fan fault is frequently the fan being made to work against a restriction rather than a fan that has died.

So when an engineer arrives, the question worth asking is not what part is broken. It is what made this part fail. A visit that swaps the component named by the code and leaves without answering that question tends to be a visit you pay for twice.

The small number under the code matters

On Worcester's newer displays there is a three digit number underneath the alert code, and their own user instructions ask you to quote both when you call, giving EA and 227 as the example. That second number narrows the fault considerably for the person on the technical line, and almost nobody reads it out because almost nobody knows it is there.

The same applies to any scrolling text on the display. Some boilers spell the fault out in words alongside the code. Photograph the screen. It costs you nothing and it is the single most useful thing you can hand an engineer before they arrive.

Source: Worcester Bosch, Greenstar i ErP user instructions 6 720 806 946

Resetting: once, and only when you know why

A lockout is a safety feature. The boiler has shut itself down rather than run in a condition it cannot verify, and the reset button asks it to try again.

Vaillant put the rule better than anyone: before you reset, make sure the cause has been resolved, and aside from very minor fixes such as thawing a condensate pipe, bleeding radiators or topping up pressure, all errors should be looked at by a Gas Safe registered engineer. Resetting without knowing why simply asks a boiler that has already refused once to refuse again.

Even the mechanics differ. On a Greenstar i you press reset briefly, once. On a Greenstar CDi Classic you hold it for three seconds. On a Vaillant it is around three seconds. Generic advice tells people to hold the button down, which is why so many boilers get held for far longer than any manufacturer asks for. Use your own manual.

And some codes cannot be cleared by a reset at all. Worcester say so plainly in their own user instructions: because of the internal safety systems, some codes will not clear, and you should contact them. If the code returns immediately, or returns for the third time, stop pressing the button. Repeat resets into a genuine overheat or a genuine ignition fault do damage, and they cost you the one piece of evidence an engineer wants, which is how the boiler behaved when it failed.

Source: Vaillant, how to reset a boiler

What you can safely check yourself, whatever the code says

The safe list is short and it is the same on every brand. None of it involves the gas circuit and none of it involves taking the case off.

• The pressure gauge. Most boilers want somewhere around 1 to 1.5 bar cold, and your manual will tell you the figure for yours. Topping up is a homeowner job.
• The condensate pipe, if the weather is cold and the pipe runs outside or through a garage. Worcester's own guidance is to thaw it with a hot water bottle, a microwaveable heat pack or warm water poured on the outside of the pipe, never boiling water, and only where the pipe is at ground level and easy to reach. Never at height.
• Radiators that are cold at the top, which usually just want bleeding.
• The obvious supply questions. Are other gas appliances working. Is there credit on a prepayment meter. Has the fused spur or the programmer been switched off by someone.
• The flue terminal, looked at from the ground. A nest, a bin pushed against it, sheeting blown over it. Look, note what you see, and do not clear anything at height.

Then stop. If you can smell gas, do not touch anything electrical, get everyone out and call the National Gas Emergency Service on 0800 111 999 from well away from the building.

Where it stops being your job

At the case. Worcester head their entire error code table with the instruction not to try to rectify any of the listed faults yourself, and to contact a competent, qualified Gas Safe registered engineer. Their user instructions say elsewhere that only a competent engineer may remove the appliance case and carry out work, in accordance with the Gas Safety (Installation and Use) Regulations. That is not a manufacturer being cautious about its warranty. It is the law about who may work on a gas appliance.

Before you phone, write down five things: the code and any number under it, the make and model from the data plate, what the boiler was doing when it locked out, whether a reset cleared it and how long it stayed cleared, and when it was last serviced. That last one matters more than people expect.

Source: Gas Safe Register; Worcester Bosch, Greenstar CDi Classic user instructions 6 720 812 179

Who pays for the visit, and why the paperwork decides it

Worcester publish their own conditions, and they are typical of the industry. A call out charge is made where the boiler is outside the guarantee period, where evidence cannot be provided that the first year service inspection was carried out, where their engineer finds no fault with the boiler, or where the cause of the breakdown is misuse or lies with another part of the heating system rather than the appliance.

Read that middle one again. A boiler still inside its guarantee period can still generate a charge if the Benchmark checklist in the back of the manual has no service entry in it. The single most valuable document in your house right now is that little booklet.

And the fourth condition explains a lot of unhappy visits. Worcester state that no boiler fault is found on over 30% of all service calls. Sometimes the code was pointing at the system, not the appliance.

Source: Worcester Bosch, Greenstar CDi Classic user instructions 6 720 812 179

When a code is really telling you to replace the boiler

One code is an event. A pattern of codes is a diagnosis, and the diagnosis is usually about the age of the appliance rather than the part named on the screen.

Vaillant say it directly in their own homeowner advice: if you find you regularly need to reset your boiler, that should not be the case, and you should get someone to find out why. Worcester make the same point from the other end, noting that a boiler frequently going into lockout may signal deeper underlying issues. Neither of those is a sales line. It is the manufacturers telling you that repeated lockouts are the appliance failing in slow motion.

Three questions settle it, and none of them are about the code itself. How old is the boiler, against the ten to fifteen years most of them manage. How big is this specific repair, because once a heat exchanger or a gas valve is involved on an out of warranty boiler you are into money that overlaps a replacement. And how many times have you met a code in the last two winters. Three call outs in two years on a twelve year old boiler is a decision, not a run of bad luck.

The other half of the answer is what cover you actually have. Manufacturer guarantees lapse for missed services, exclude damage from sludge and scale, and in some cases run from the date of manufacture rather than the date of installation if the paperwork is missing. An older boiler is often less protected than the certificate in the drawer suggests.

Source: Vaillant, how to reset a boiler; Worcester Bosch, resetting your boiler when it locks out

The codes we have written up so far

Each of these follows the same shape: what the code means in the manufacturer's own words, the causes in the order they usually turn out to be, what is safe for you to check, where it stops, and whether it is a repair or a replacement signal.

If the codes have stopped being occasional

Around Northampton and Milton Keynes we see the same pattern every autumn. Whole streets of estate stock were fitted at the same time, so they age at the same time, and the first cold snap brings out the condensate lockouts and the overheat codes together. If your boiler has been resetting itself into another winter for a couple of years, it is worth getting the replacement priced while you still have heating, rather than in the week it finally stops.

Tell us about the job and we will put you in touch with three vetted local installers. There is no obligation, and a planned replacement is always a calmer piece of work than an emergency one.