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Worcester Bosch Boiler Faults & Error Codes Guide
Worcester Bosch Greenstar boilers are reliable, but like any combi they develop faults. This hub explains the most common Worcester problems, how to read the error codes, the checks you can safely make yourself, and the point where you need a Gas Safe registered engineer.
Quick answer
The most common Worcester Bosch Greenstar faults are low water pressure, no hot water (often a stuck diverter valve or flow sensor), ignition or lockout faults showing a code such as EA, a frozen condensate pipe, fan/pump/overheat faults (codes like C6 or E9), and no power or response.
You can safely check the power and controls, top up the pressure via the filling loop, try one front-panel reset, and thaw a frozen condensate pipe.
Anything inside the sealed boiler — and all gas work — is for a Gas Safe registered engineer, and you should reset a lockout only once.
The most common Worcester Bosch faults
Worcester Bosch is one of the UK's biggest boiler brands, and most homes run a Greenstar combi, system or heat-only model. The faults we see most often are rarely catastrophic — they tend to be the same handful of issues across the range. Knowing which is which helps you decide whether it's a five-minute fix or an engineer's job.
- Low water pressure — the gauge drops below about 1 bar and the boiler locks out. Usually a slow top-up cures it, though a persistent drop points to a leak.
- No hot water (but heating still works) — often a stuck diverter valve, a flow sensor, or the comfort/preheat setting being changed.
- An ignition or lockout fault — the boiler tries to fire, fails, and shows a code such as EA, then shuts down as a safety measure.
- A frozen condensate pipe — in cold snaps the external white plastic waste pipe ices up and the boiler stops, often with a gurgling sound.
- Fan, pump or overheat faults — internal mechanical faults that show codes like C6 (fan) or E9 (overheat) and need parts replacing.
- No power or no response — a tripped fuse, a flat thermostat battery, or the programmer not calling for heat.
Beyond these, a handful of other faults turn up regularly — a noisy boiler or kettling in hard-water areas, a pilot light going out on older heat-only units, a boiler leaking water, and overheating. We cover each below, with the dividing line between a safe homeowner check and a Gas Safe engineer's job made explicit every time.
The six headline faults in detail
Each of the most common Worcester problems below follows the same shape: the symptom you'll notice, the likely cause, what's genuinely safe to check yourself, and the point where it becomes Gas Safe engineer work. None of the safe checks involve gas, the flue, the fan, the sealed combustion circuit or removing the casing — and they never should.
1. Low water pressure
Symptom: the pressure gauge (or digital reading) sits below about 1 bar, the heating won't fire, and you may see a low-pressure warning or code (the exact code varies by model, so check your manual). Likely cause: a gradual loss of system water — often a tiny weep on a radiator valve or pipe joint, air being bled from radiators, or, if it keeps dropping, a leak or a failed expansion vessel. Safe to check: read the gauge cold (it should be roughly 1–1.5 bar), then top up via the filling loop as described below. Gas Safe work: a pressure that keeps falling within days of a top-up points to a leak or an expansion-vessel fault that needs an engineer — repeated topping up just masks it.
2. No hot water (heating still works)
Symptom: radiators heat up fine but the taps run cold, or hot water is weak and short-lived. Likely cause: on a combi, a stuck diverter valve (which should switch the boiler from heating to hot water when you open a tap), a faulty DHW flow sensor or thermistor, or simply the comfort/preheat setting having been changed. Safe to check: confirm the hot-water setting and the comfort/eco mode on the front panel haven't been turned down or off. Gas Safe work: a diverter valve, flow sensor or thermistor is an internal part — replacing it means taking the casing off and is engineer-only.
Our Worcester boiler no hot water guide goes deeper.
3. Ignition or lockout fault (EA)
Symptom: the boiler tries to fire, fails, and shows a lockout code such as EA, often with a flashing light. Likely cause: the PCB couldn't detect a flame after its ignition attempts — typically a worn ignition electrode, a dirty or sooted flame sensor, a gas-supply issue or a blocked condensate pipe. Safe to check: press the reset button once and watch what happens; check other gas appliances are working (which rules out a supply-wide problem). Gas Safe work: if the EA returns, stop.
The electrode, gas valve, flame sensor and combustion circuit are all engineer-only.
4. Frozen condensate pipe
Symptom: the boiler stops in a cold snap, often with a gurgling sound and a lockout code (commonly EA 229 — loss of an established flame caused by a blocked or frozen condensate). Likely cause: the external white plastic waste pipe (a narrow drain pipe, not to be confused with the larger flue) has iced up, so the boiler can't drain its condensate and locks out. Safe to check: pour warm — not boiling — water over the frozen external section to thaw it, then reset once. Gas Safe work: if it keeps refreezing or the trap inside the boiler is blocked with debris, that's an engineer's job.
See our frozen condensate pipe guide.
5. Fan, pump or overheat fault
Symptom: a lockout showing a code such as C6 (fan) or E9 (overheat), sometimes after a period of noisy running. Likely cause: the combustion fan isn't reaching its expected speed (C6), or the boiler has detected too high a temperature — usually poor circulation, a stuck pump or sludge (E9). Safe to check: nothing internal; you can confirm pressure is correct, as very low pressure can affect circulation. Gas Safe work: the fan, pump and the components behind an overheat are all sealed inside the boiler and are engineer-only.
6. No power or no response
Symptom: a blank display, no firing at all, or the boiler ignoring the thermostat. Likely cause: a tripped fuse or RCD, a flat wireless-thermostat battery, the fused spur being switched off, or the programmer not actually calling for heat. Safe to check: the consumer unit, the fused spur, the thermostat batteries, and that the programmer is set to "on" and calling for heat. Gas Safe work: if power is confirmed but the boiler still won't respond, the fault is likely internal (PCB or wiring) and needs an engineer.
Other common Worcester faults
Beyond the six above, these four turn up often enough to deserve their own note — and the gas-safety boundary matters just as much here.
Noisy boiler or kettling
Symptom: a rumbling, whistling or banging sound — like a kettle coming to the boil — usually from the boiler itself rather than the radiators. Likely cause: in hard-water areas, limescale (calcium carbonate) builds up on the heat exchanger, restricting water flow so it overheats and produces steam; sludge can do the same.
This is more common the longer a boiler has been in a hard-water region. Safe to check: nothing inside the boiler — kettling is a symptom of scale or sludge in the sealed circuit. Gas Safe work: a power flush, a chemical descale, or fitting a scale reducer or magnetic filter are all engineer jobs.
Persistent kettling shortens a heat exchanger's life, so don't ignore it. Our guide to boiler kettling noise explains the fix.
Pilot light going out (older heat-only units)
Symptom: on an older Worcester unit with a standing pilot, the small permanent flame keeps going out, so the boiler won't fire. (Modern Greenstar condensing boilers use electronic ignition and have no pilot light — if yours shows an EA code instead, see the ignition fault above.) Likely cause: a faulty or dirty thermocouple — the safety device that senses the pilot and holds the gas valve open — carbon deposits, or a draught. Safe to check: nothing; the pilot, thermocouple and gas valve are all part of the gas circuit. Gas Safe work: because this is inherently gas work, a relight or thermocouple replacement must be done by a Gas Safe registered engineer.
Boiler leaking water
Symptom: water pooling under or around the boiler, often alongside dropping pressure and a lockout. (A leak commonly shows up indirectly as a pressure-loss or pump fault — codes like EA 338 or A1 281 — rather than a dedicated "leak" code, so confirm the meaning against your manual.) Likely cause: a perished seal, a worn pump seal, a failed valve, or — if pressure has been running high — a weeping pressure relief valve. Safe to check: turn the heating off, mop up the water and note where it's coming from so you can describe it; that's the limit. Gas Safe work: never run a leaking boiler long-term, and don't open the casing.
Internal seals, the pump, valves and the heat exchanger are all engineer-only.
Boiler overheating
Symptom: the boiler cuts out under load and shows an overheat lockout (commonly E9), sometimes after kettling noises. Likely cause: poor circulation from a stuck pump, sludge or limescale, a blockage, or a faulty temperature sensor letting the water get too hot. Safe to check: confirm the system pressure is correct, as very low pressure reduces circulation. Gas Safe work: the pump, sensors, heat exchanger and the cause of any blockage are inside the sealed unit — diagnosis and repair are engineer-only.
Don't keep resetting an overheating boiler.
Worcester Bosch error codes at a glance
Modern Greenstar boilers show a fault as a short code on the front display — a letter group followed by a number, such as EA, E9 or C6. The table below is a quick reference, with the symptom tied to the code where Worcester's own numbering narrows it down.
The exact suffix digits and meanings vary between the Greenstar i, CDi and older Si ranges, so always cross-check against your manual or the label inside the boiler's casing flap.
| Code | What it usually means | Who should deal with it |
|---|---|---|
| EA | Ignition / flame fault — the boiler couldn't detect a flame and locked out. | Reset once; Gas Safe engineer if it returns · EA guide |
| EA 229 | Loss of an established flame — most often a frozen or blocked condensate pipe (the flame lit but then went out). | Thaw the condensate; engineer if it recurs · EA guide |
| EA 338 / A1 281 | A low-pressure or pump-circulation lockout — the boiler isn't detecting enough water flow. A hidden leak is one common underlying cause. | Repressurise first; engineer if it recurs · A1/281 guide |
| E9 | Overheat — too high a temperature detected, often poor flow or a stuck pump. | Gas Safe engineer · E9 guide |
| C6 | Fan fault — the combustion fan isn't running at the expected speed. | Gas Safe engineer · C6 guide |
| A1 | No water flow — often a seized pump or an airlock. | Check pressure first; engineer for the pump · A1/281 guide |
| F0 | Internal / PCB fault the boiler can't categorise more precisely. | Gas Safe engineer · F0 guide |
| Low-pressure warning / code | Pressure has dropped below the safe minimum (around 1 bar). The exact code varies by model — check your manual. | Homeowner can repressurise |
| H13 (and other H-codes) | A service reminder, not a fault — a set service interval has elapsed. The boiler keeps working; book a service. | Book an annual service |
Blocking errors vs locking errors
Worcester splits faults into two types, and the difference tells you what to expect:
- Locking errors shut the boiler down completely and need a manual reset to clear. You'll see a flashing fault code (often with a warning-triangle symbol) — EA is the classic example. These are the "reset once, then call an engineer if it returns" faults.
- Blocking errors stop the boiler temporarily but can clear on their own once the cause goes away — for instance a brief overheat or flow issue. They may not display a code on the main screen the same way; the detail can sit in the boiler's info menu.
H-codes are different again: codes beginning with H — such as H13 — are maintenance reminders rather than faults. They flag that a set service interval has elapsed and the boiler is due a service; it carries on running. Book your annual service rather than treating it as a breakdown.
The same code can mean different things by model
Worcester uses separate code tables for each Greenstar range, and the suffix digits in particular differ between models — so don't read across from a code you found for a different unit. The note below is a rough orientation only; the full code list lives on our dedicated Worcester Bosch error codes page, and your manual is always the final word.
| Model range | What to know about its codes |
|---|---|
| Greenstar i (e.g. 8000 Life, 4000, i Compact) | Newer text/symbol displays; codes such as EA with a numeric suffix (EA 229, EA 338) that pinpoint the cause. |
| Greenstar Si Compact | An older-generation compact combi; same letter groups, but suffix digits can differ from the i range. |
| Greenstar CDi range (Classic, Compact, Highflow) | Long-running range; check your specific CDi model's table — codes are not interchangeable with the i range. |
| 8000 Style | Premium current model with a colour display that often spells the fault out in words as well as a code. |
For the full list with plain-English explanations of each code, see our dedicated guide to Worcester Bosch error codes.
What you can safely check yourself
Only a short list of Worcester faults is genuinely a homeowner job. None of these involve gas, the flue, the sealed combustion circuit or removing the casing — and they never should. Anything beyond this list is Gas Safe engineer work only.
1. Check the power and controls
Make sure the boiler has power, the fused spur is switched on, and any wireless thermostat or programmer isn't flat or dropped out of its settings. A tripped fuse or a flat thermostat battery can stop the boiler responding entirely — both are safe to check and put right.
2. Check (and top up) the pressure
Find the pressure gauge or digital reading on the front. Cold, it should sit at roughly 1 to 1.5 bar, rising towards about 2 bar when hot. Below 1 bar is low.
You can top up via the filling loop — the braided silver hose (or internal keyed valve) underneath the boiler — opening the valves slowly until the gauge reaches about 1.5 bar, then closing them.
If the pressure keeps dropping, you have a leak that needs investigating rather than endless topping up. Our guide to low boiler pressure walks through it.
3. Try one front-panel reset
If a lockout code such as EA is showing, press and hold the reset button on the front panel for a few seconds — once. If the boiler fires and stays running, keep an eye on it. If the same code returns, stop resetting and book an engineer. Our step-by-step guide on how to reset a Worcester Bosch boiler covers the exact button sequence.
4. Thaw a frozen condensate pipe
In freezing weather, the external white plastic condensate pipe can ice up and stop the boiler. You can safely pour warm — not boiling — water over the frozen section to thaw it, then reset once. See our guide to a frozen condensate pipe for the full method.
When you need a Gas Safe registered engineer
If the safe checks above don't fix it, the cause is almost certainly internal — and these are not DIY jobs.
Anything involving the gas valve, the burner, the flame-sensing electrode, the flue, the sealed combustion circuit, the pressure relief valve, or removing the casing must be done by a Gas Safe registered engineer. It is illegal as well as dangerous for an unregistered person to attempt gas work.
Book an engineer for:
- An error code that returns after a single reset.
- A stuck diverter valve, DHW flow sensor or thermistor causing no hot water — see our Worcester boiler no hot water guide.
- Fan (C6), pump or overheat (E9) faults that need parts replaced.
- A PCB or internal sensor fault (F0-type codes).
- Pressure that keeps dropping, suggesting a leak or expansion-vessel fault.
You can confirm any engineer is genuinely qualified at the Gas Safe Register. (Gas Safe replaced the old CORGI scheme in 2009, so always look for the Gas Safe ID card, not "CORGI registered".) For what these jobs typically cost, see our Worcester boiler repair guide.
What these repairs typically cost
Once a fault needs an engineer, the bill is mostly the call-out plus the part. These are indicative ranges, last checked 2026 — they vary by region, by model and by how urgently you need someone out, so treat them as a guide, not a quote. A diagnostic visit is usually from around £80.
| Typical job | Indicative cost (2026) |
|---|---|
| Diagnostic / call-out | from ~£80 (£70–£120) |
| Flame-sensing electrode (EA-type faults) | £100–£180 |
| Diverter valve (no hot water) | £200–£400 |
| Combustion fan (C6) | £200–£450 |
| Pump (overheat / flow faults) | £200–£400 |
| Gas valve | £250–£500 |
| PCB / control board (F0-type faults) | £300–£600 |
| Heat-exchanger descale / power flush (kettling) | £300–£600 |
For a fuller breakdown by part and job, see our Worcester boiler repair guide.
How boiler cover helps with Worcester faults
The pricier Worcester repairs — a fan, a pump, a gas valve or a PCB — can each run to several hundred pounds in parts and labour, often more than a year of cover. A boiler cover policy turns that into a fixed monthly amount and gives you a number to call when the heating or hot water goes.
If you're weighing it up, our guides to what boiler cover is, the best boiler cover and whether boiler cover is worth it set out what's actually included. Always check the boiler-age limit, the excess and any limescale or pre-existing fault exclusions before you buy.
It pays to compare cover across our panel — prices and cover levels vary more than most people expect.
One Worcester repair can cost more than a year of cover
A single fan, pump or gas-valve job can run to several hundred pounds. Compare boiler-cover plans side by side and see what a fixed monthly premium would protect you against.
Compare boiler coverFrequently asked questions
What is the most common Worcester Bosch boiler fault?
Low water pressure is the one we see most, followed by no hot water (usually a diverter valve or comfort setting) and ignition lockouts showing an EA code. Many are homeowner-fixable; the rest need a Gas Safe registered engineer.
Can I clear a Worcester Bosch fault myself?
You can reset a lockout once from the front panel, repressurise a low-pressure fault via the filling loop, and thaw a frozen condensate pipe. If the fault returns after one reset, stop — the remaining causes need an engineer with the casing off.
What does a flashing blue light on my Worcester boiler mean?
A flashing light on the front panel usually goes hand in hand with a fault code, most often an EA ignition or flame fault — so read the code on the display rather than the light alone. A steady light generally means normal standby.
Try a single reset; if the light keeps flashing and the code returns, book a Gas Safe registered engineer rather than resetting repeatedly. The exact light behaviour varies by model, so check your manual.
Why does my Worcester boiler keep losing pressure?
A pressure gauge that keeps dropping usually means a leak somewhere in the system or a failed expansion vessel. Topping up via the filling loop is a short-term fix only — if it recurs over days, have it investigated rather than refilling repeatedly.
How many times should I reset my Worcester boiler?
Once. If the fault clears and the boiler stays running, fine. If the same code comes straight back, don't keep resetting — book a Gas Safe registered engineer, because repeated resets won't cure the underlying fault.
Will boiler cover pay for a Worcester Bosch repair?
Most heating-repair policies cover parts and labour for faults like these, subject to the boiler being in good working order when you took the policy out, the boiler-age limit, and any excess. Always check the exclusions before you buy.
This article is general information, not advice, and reflects a selected panel of providers rather than the whole market. Prices and ranges are indicative for 2026. Always use a Gas Safe registered engineer for gas work.