Worcester Bosch EA 338 fault — what it means and how to fix it

What does the Worcester Bosch EA 338 fault mean?

The Worcester Bosch EA 338 fault means your boiler tried to run but locked out because the system water pressure is too low. In plain terms: the boiler can fire up, but it can't sense enough water flowing through it, so it shuts itself off to protect the heat exchanger rather than run dry.

The good news is that EA 338 is one of the friendlier faults to see. Most of the time it's fixed in a few minutes by topping the pressure back up yourself — and you can do that safely without touching anything to do with gas.

Quick note on the code itself: "EA" is Worcester Bosch's flame/ionisation lockout family, and the three-digit number after it tells you which part of the sequence stopped. EA 227 is the no-flame version — read our EA / 227 guide if that's what you're seeing. EA 338 typically presents as the low-pressure lockout: the boiler can ignite but locks out because it can't confirm enough water in the system. The exact wording and trigger vary by Greenstar model, so it's always worth checking it against your boiler's manual.

One thing to clear up: EA 338 on its own is not a gas emergency. The National Gas Emergency line, 0800 111 999, is only for when you can smell gas or your carbon monoxide alarm is going off — not for this code.

EA 338 at a glance — the key facts

Why does my boiler keep cutting out and showing EA 338?

If your boiler fires up, runs for a few minutes, then cuts out and shows EA 338 again, the pressure has dropped below the level it needs to run safely. The boiler keeps trying, keeps coming up short of water, and keeps locking out to protect itself.

The most common reason is simply that the system has slowly lost a little water over time — radiators, valves and joints all let a tiny amount go, and that's normal. Topping it up once or twice a year is nothing to worry about. Worcester Bosch's own guidance says the same: an occasional top-up is fine; needing it often is a sign something needs looking at.

If you top it up and it holds, you were probably just due a top-up. If it drops again within a day or two, you've got a leak somewhere or a part that's failing — and that's the point to stop topping up and call someone out. We'll come to that fork below.

What causes the EA 338 fault?

EA 338 is a low-pressure lockout, so anything that lets water out of the sealed system — or stops it holding pressure — can trigger it. Some causes are a one-off; others come back until they're fixed. The table below covers them, roughly most-likely first.

Why does my boiler lose pressure after I top it up?

If the gauge drops again soon after a top-up, three things are usually behind it. The most common is a small leak — a pinhole in a radiator, a weeping valve, or a joint under the floor. The second is a failed expansion vessel: when its air charge is gone, you'll see the pressure shoot up fast as the system heats and drop sharply as it cools. The third is the pressure relief valve "letting by" — water dribbling out of the small pipe that pokes through an outside wall, often missed before people start pulling up floorboards.

Whichever it is, the answer isn't to keep topping it up. That only masks the problem, and constantly adding fresh water can introduce air and sludge that wears the system over time. Find the cause, or get an engineer to.

EA 338 causes and symptoms

How do I check and fix the EA 338 fault myself?

There's a clear safe job you can do here: check the pressure and top it back up. None of it involves opening the boiler or touching anything to do with gas — that's the line you never cross.

How do I check if my pressure is too low?

Look at the pressure gauge on or under the boiler — it's a round dial, or a digital reading on newer models. When the system is cold it should sit somewhere between 1.0 and 1.5 bar, with about 1.3 bar being ideal. If the needle is sitting below 1 bar, or down in the red zone, that's almost certainly what's causing the EA 338 lockout.

How do I top up the pressure using the filling loop?

The filling loop is how you add water back into the system. On most Worcester Bosch models it's the braided silver hose underneath the boiler with a valve at each end; on some newer Greenstars it's a built-in key or lever instead. The job is simple, but go slowly.

1. Turn the boiler off and let it cool, so you're topping up against a true cold reading.

2. Find the filling loop — the silver braided hose (or the integral key/lever) under the boiler.

3. Open both valves slowly. You'll hear water flowing in and the gauge will start to rise.

4. Watch the gauge and stop at about 1.0 to 1.5 bar (aim for around 1.3). Don't go past 2 bar.

5. Close both valves firmly. Wait ten to fifteen minutes, then reset the boiler once and check it fires up and stays on.

I've added too much pressure — what do I do?

If you overshoot past about 2 bar, don't panic. Bleed a little water off through a radiator bleed valve — open it with a radiator key for a moment with a cloth and a cup ready to catch the drips, then close it again. Watch the gauge settle back to around 1.3 bar. Going a touch over 2 bar isn't dangerous — the boiler's pressure relief valve doesn't actually open until around 3 bar — but it's still better to bleed the excess down yourself than to let the pressure climb, so the system isn't sitting higher than it needs to be.

How do I clear the EA 338 code once the pressure is right?

Once the gauge is sitting at around 1.0 to 1.5 bar cold, press and hold the reset button (the flame symbol, or the button marked "reset") for about three seconds, then let go. The boiler will run its start-up sequence and, if the pressure's now correct, fire up normally. The exact button varies by model, so check your manual if you're not sure which one it is.

Reset once — and only once. If EA 338 comes straight back even though the gauge reads a healthy pressure, the issue may be the pressure sensor rather than the water level, and that's a Gas Safe registered engineer's job. Don't sit there resetting it.

When should I call a Gas Safe registered engineer?

Here's the simple fork. If you top up and the pressure holds steady over a day or two, you're fine — it was a one-off top-up and you don't need anyone out. If the pressure drops again within hours or days, stop topping it up and book an engineer: that's a leak, a failed expansion vessel or a PRV letting by, and those are all engineer-only fixes. Tracing leaks inside the boiler, recharging or replacing the expansion vessel, and swapping the PRV or pressure sensor all sit on the gas side of the boiler — it's illegal for anyone who isn't Gas Safe registered to do that work. You can check an engineer is registered on the Gas Safe Register.

While you wait, you can book a one-off repair with us and one of our Gas Safe registered engineers will come and fix it for you.

How do I find the leak causing EA 338?

Before anyone pulls up floorboards, there are obvious spots worth a look. Check under and behind every radiator and towel rail for damp patches or rust marks, feel around the radiator valves, and look at the joints near the boiler itself. Then go outside and check the small pressure relief pipe that pokes through the wall — if it's dripping or has left a stain, that's your culprit and it's a job for an engineer, not a hidden pipe.

Some leaks are genuinely hidden — under floors, inside walls, or a slow weep on a buried joint. If you can't see anything but the pressure still falls, don't keep guessing. A Gas Safe registered engineer can pressure-test the system and trace it properly, and that's far cheaper than chasing it blind.

EA 338 vs EA 227 and EA 229 — what's the difference?

These look similar but point to different things, which is why people get them mixed up. The "EA" is the family; the number tells you which part of the sequence failed.

EA 227 means no flame was detected — the boiler tried to light and couldn't sense a flame, so it locked out. EA 229 means the flame was established and then lost partway through a burn. EA 338 is the one this guide covers: the boiler can fire but locks out on low water pressure. If your gauge is sitting low, you're almost certainly looking at EA 338. If the boiler is clicking to ignite but never "whooshes" into life, that's the EA 227 territory instead — and we've got a separate guide for that.

Worth knowing: which exact number your Greenstar shows depends on its model and software, and a couple of guides online disagree about what EA 338 means. If your gauge clearly reads low, treat it as a pressure fault and follow the steps above. If the gauge looks fine and EA 338 still shows, have a Gas Safe registered engineer read the code on the unit and confirm it — the safest way to be sure on your exact model.

How much does it cost to fix EA 338?

It depends entirely on the cause, so the honest answer is a range. The figures below are typical UK industry costs to give you a feel for it — they are not Smart Plan's prices, and your own quote will depend on your boiler and where you live.

If it's a one-off top-up, it costs you nothing — you've just done it yourself. From there it's a ladder. A diagnostic or leak-detection call-out is usually around £60–£120. A pressure sensor swap is at the cheaper end, often grouped with the call-out. Recharging or replacing an expansion vessel is typically £100–£200, and replacing the pressure relief valve is in a similar bracket. A new PCB is the dear one, often £300–£500 and sometimes higher if water has got into it.

Should you repair or replace the boiler?

A top-up or a small leak fix is worth doing at almost any age. But if you're facing a PCB or a recurring leak on a boiler that's already 10–15 years old, it's worth getting a view on a replacement. As a rough rule, once a repair creeps toward a third or more of the cost of a new boiler, replacing it often makes more sense than stacking up repairs. A Gas Safe registered engineer can tell you which way to lean.

Smart Plan is a service plan, not insurance. If you'd rather not face a surprise repair bill next time the pressure plays up, an ongoing boiler module covers parts and labour up to your cover limit, so a fault like this is sorted for you. Cover is modular — you only pick what you want, so you don't pay for what you don't use. Boiler cover runs up to £500 per year if your boiler's under 7 years old, or up to £200 if it's older. A £95 call-out fee applies. We've looked after over 15,000 customers, we're from UK Boiler Company Ltd, trading since 2014, and we're rated on Trustpilot.

Safe checks before you call anyone out

The only things worth trying yourself with an EA 338 fault. If the pressure won't hold, the next step is a Gas Safe registered engineer.

• Check the pressure gauge — it should sit around 1.0 to 1.5 bar when cold.
• If it's low, top up via the filling loop, opening both valves slowly, then close them firmly.
• Aim for around 1.3 bar and don't go past 2 bar — if you overshoot, bleed a little off through a radiator bleed valve rather than leaving it high.
• Reset the boiler once — just once. Don't keep resetting if EA 338 comes back.
• Look under radiators and valves for damp patches, and check the small relief pipe outside for drips.
• If the pressure drops again within a day or two, stop topping up — that's a leak or a failed part for an engineer.
• Never open the casing or touch the gas valve, sensor or wiring — that's gas work, and it's a Gas Safe registered engineer's job.
• If you ever smell gas or your CO alarm sounds, stop. Call 0800 111 999, open windows, turn off the gas at the meter, don't touch electrical switches, and leave the house.

Worcester Bosch EA 338 fault FAQs

Pressure keeps dropping? We'll come and fix it for you.

Book a one-off repair and a Gas Safe registered engineer will trace the leak, or set up an ongoing Smart Plan boiler module so the next fault's covered. A service plan, not insurance — parts and labour included up to your cover limit.

Helpful links

• EA / 227 fault — no flame detected
• All Worcester Bosch fault codes
• No heating? Book a repair
• Boiler & central heating cover
• More boiler guides on the blog
• Check an engineer on the Gas Safe Register