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A code flashing on your Greenstar display tells you why the boiler has stopped. Here's a plain-English reference for the most common Worcester Bosch codes, which ones you can safely deal with yourself, and which are strictly a Gas Safe engineer's job.
Worcester Bosch is one of the UK's most common boiler brands, and most modern Greenstar combi, system and heat-only models show a fault as a short code on the front display. Older units flash a number or letter; newer ones with a text display (and the EasyControl app) often spell out the issue too.
The codes usually come as a letter category followed by a number — for example EA, E9 or C6. The letter hints at the type of fault (an "E" or "EA" group around ignition and flame, a "C" group around the fan and air, an "A" group around water flow and sensors). The exact code, and especially the suffix digits, can vary between models and generations, so always cross-check against the manual or the sticker inside your boiler's casing flap for your specific unit.
The table below lists codes you're most likely to see, what each one means in plain English, and whether it's something a homeowner can safely act on or a job for a Gas Safe registered engineer. Treat the cost and detail as general guidance, not a diagnosis — your manual is the final word for your model.
| Code | What it means | Who should deal with it |
|---|---|---|
| EA | Ignition / flame fault — the boiler tried to fire but no flame was detected (or the flame signal was lost), so it has locked out as a safety measure. | Gas Safe engineer (one reset only — see below) |
| E9 | Overheat — the boiler has detected too high a temperature and shut down to protect itself. Often poor flow, a stuck pump or a sensor fault. | Gas Safe engineer |
| C6 | Fan fault — the combustion fan isn't running at the expected speed, or its signal isn't being read correctly. | Gas Safe engineer |
| EA / low-pressure codes (e.g. CE 207, F0/low water) | Low system pressure — the boiler can't fire because water pressure has dropped below the safe minimum (around 1 bar). | Homeowner can repressurise (see below) |
| D1 | External sensor or outdoor-sensor fault — typically a wiring or weather-compensation sensor issue rather than a combustion problem. | Gas Safe engineer |
| F0 | Internal fault — a general electronics / control-board (PCB) or internal sensor error the boiler can't categorise more precisely. | Gas Safe engineer |
| A1 | Pump / no-water-flow fault — the boiler isn't sensing enough circulation, often a seized pump or an airlock. | Gas Safe engineer (you can first check pressure) |
| F7 / FA | Flame-detection / residual-flame fault — the flame sensor is giving an unexpected reading, so the boiler shuts down. | Gas Safe engineer |
If your code isn't in this list, don't guess from a similar-looking one. The same two characters can mean different things across the Greenstar i, Greenstar CDi and older Greenstar Si ranges. Check your model's manual or the Worcester Bosch support pages for the exact meaning.
Only a short list of these is genuinely DIY. Everything that touches the gas valve, the burner, the flame-sensing electrode, the flue or the sealed combustion circuit means taking the casing off and using test equipment — that is Gas Safe engineer work only, and it is illegal as well as dangerous for an unregistered person to attempt it.
The homeowner-safe checks are:
For a lockout code like EA, you can press and hold the reset button on the front panel for a few seconds to restart the ignition sequence. Do this only once.
If you have a low-pressure code, repressurising via the filling loop is a safe homeowner job. The gauge should read about 1 to 1.5 bar when cold, rising towards roughly 2 bar when hot. Below about 1 bar is low.
If the pressure keeps falling over days or weeks, you have a leak or an expansion-vessel fault that needs an engineer — not endless topping up. For more detail, see our guide to low boiler pressure.
If a code returns after a single reset, you'll need a professional repair. These are indicative 2026 ranges and vary by region, parts and call-out timing.
| Job | Indicative cost |
|---|---|
| Engineer diagnostic / call-out | £70 – £120 |
| Replace flame-sensing electrode (EA-type faults) | £100 – £180 |
| Replace combustion fan (C6) | £200 – £350 |
| Replace pump (overheat / flow faults) | £200 – £400 |
| Replace gas valve | £250 – £450 |
| PCB / control board (F0-type internal faults) | £300 – £500+ |
This is where a policy earns its keep. With boiler cover, a repair like a failed fan or gas valve is handled for the price of your monthly premium instead of a one-off bill. If you're weighing it up, our guides to the best boiler cover and whether boiler cover is worth it break down what's actually included — and you can compare cover across our panel in a couple of minutes.
A single fan, pump or PCB 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 coverWhen a Worcester Bosch boiler locks out it stops firing, so there's no immediate hazard from the lockout itself — but you'll have no heating or hot water until it's fixed. If you ever smell gas, don't reset it: call 0800 111 999 and leave the property.
You can reset a lockout once from the front panel, repressurise if it's a low-pressure code, and thaw a frozen condensate pipe. If the code returns after one reset, stop — the remaining causes (electrode, fan, gas valve, flue, PCB) need a Gas Safe registered engineer with the casing off.
A repeating code means the underlying fault hasn't been fixed — a worn electrode on an EA, a failing fan on a C6, or a flow problem on an E9. Repeated resets won't cure it, so get it diagnosed rather than cycling the reset button.
Yes. The exact digits after the letter group vary by model and generation across the Greenstar range, and they narrow down the cause. Always check the code against the manual or the label inside your boiler's casing flap for your specific unit.
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 on the plan. Always check the exclusions before you buy.