Worcester Bosch Greenstar fault codes
If your Greenstar boiler is showing an error code, this sets out what it means, which part is most likely at fault, and whether the fix is one you can do yourself or one for a Gas Safe registered engineer.
Browse fault code guides and parts articlesIdentifying your Worcester Bosch model range
Worcester Bosch has sold several Greenstar ranges over the years, and the range your boiler belongs to changes what a fault code means and which part fits it. A code that points to a diverter valve on one range can point to a different assembly on another, so working out which Greenstar you have comes before working out what the display is telling you.
Why the range matters
Worcester Bosch has updated the electronics, the heat exchanger design, and the control interface across the Greenstar line more than once. Two boilers can share the Greenstar name and the same fault code number while needing a different printed circuit board (PCB), a different sensor, or a different diverter valve to fix it. Ordering by fault code alone, without checking the range, is how the wrong part ends up on the doorstep.
Greenstar i
The i range is a compact wall-hung combi line. It uses its own control board and display, and parts sourced for it are not interchangeable with the CDi or 8000 Life ranges even where the fault code looks identical on screen.
Greenstar Si
Si models are typically paired with a system boiler setup. A code relating to hot water flow, for instance, means something different on a system boiler than it does on a combi, because the two heat domestic hot water in different ways.
Greenstar CDi
CDi covers a wider spread of condensing variants across combi, system, and heat-only configurations. Because the CDi name spans several sub-models, the model plate detail matters more here than on the other ranges: two CDi boilers can differ in exactly which sensor or valve sits behind the same fault code.
Greenstar 8000 Life
The 8000 Life range is a newer generation with its own control interface and internal layout, and parts designed for the older i, Si, and CDi ranges are not assumed to fit it.
Where to check
The data badge on the inside of the boiler casing gives the model name and the GC number, which is the identifier used across the UK gas appliance industry to pin down the exact appliance variant. Have that number ready before searching for a fault code or a part, and check the physical part label against anything you're about to order. The fault code guides and parts articles cover how to read the badge details against a specific code, and the Gas Safe rules for boiler repairs page sets out which of these jobs you're permitted to carry out yourself once the part is identified.
Common Worcester Bosch Greenstar fault codes compared
These are among the fault codes Worcester Bosch Greenstar owners see most often, grouped by what the boiler is telling you, since the display text itself varies a little between ranges. Where a code points to a gas-carrying part, ignition, flame sensing or the burner sequence, the repair sits with a Gas Safe registered engineer regardless of how confident you feel opening the casing.
| Fault code | What it usually means | Likely component | Who can do the work |
|---|---|---|---|
| EA | The boiler is failing to ignite or losing ignition on startup | Ignition electrode, flame sensor, or the printed circuit board | This work must be carried out by a Gas Safe registered engineer |
| E9 | A flame was detected then lost during operation | Flame sensing electrode or gas valve | This work must be carried out by a Gas Safe registered engineer |
| D5 | System water pressure has dropped below the boiler's working range | Pressure sensor or expansion vessel | Repressurising the system is something most homeowners manage themselves; replacing the sensor or vessel means opening the casing, which a Gas Safe registered engineer should do |
| D1 | The boiler's control electronics are not completing a normal reset or communication cycle | Printed circuit board (PCB) | This work must be carried out by a Gas Safe registered engineer |
| C6 | The fan is not reaching or holding the speed the control board expects | Fan assembly or the PCB driving it | This work must be carried out by a Gas Safe registered engineer |
| F28 | Repeated ignition attempts have all failed and the boiler has locked out | Ignition electrode, gas valve, or PCB, depending on which part of the sequence is failing | This work must be carried out by a Gas Safe registered engineer |
Fault code meanings and affected parts follow Worcester Bosch service documentation as a general pattern. The exact wiring, part number and even the code itself can differ between a Greenstar i, a CDi and an 8000 Life boiler of the same era, so treat this as a starting point and check the specific code against your own model range and its printed circuit board firmware before ordering anything.
EA fault code: ignition failure explained
An EA code on a Worcester Bosch Greenstar display means the boiler tried to light and failed, or lit and then lost the flame partway through the ignition sequence. The boiler's control system waits for a signal confirming a stable flame, and when that signal doesn't arrive within its allowed window, it locks out on EA.
What usually causes it
The most common cause is a fouled or misaligned ignition electrode, the small ceramic-tipped rod that sparks to light the burner and then reads the flame to confirm it's burning. Soot, limescale, or a gap that's widened slightly over time can stop it sparking cleanly or reading the flame correctly, and the boiler reads that as a failed ignition even though gas is present.
Other common causes include a restricted gas supply, a fault with the fan or pressure switch preventing the pre-purge sequence from completing, or a printed circuit board (PCB) that isn't sending or interpreting the ignition signal correctly. This is why EA on the display doesn't point to a single part in every case.
Checking the electrode
An engineer working through this fault will usually inspect the electrode before assuming the PCB is at fault, since it's the more common failure and far cheaper to replace. That inspection covers:
- The gap between the electrode tip and the burner, which needs to sit within the tolerance set for that model range
- Visible carbon deposits or scaling on the ceramic tip
- Cracks in the ceramic insulator, which will cause the spark to track somewhere other than the burner
- The condition of the lead connecting the electrode to the PCB
A worn or dirty electrode can usually be cleaned or replaced without disturbing the rest of the ignition system, and it's the first thing to rule out before ordering anything more expensive.
Electrode or PCB: telling them apart
The electrode is a physical wear part sitting in the combustion chamber; it degrades with use and is expected to need replacement occasionally over a boiler's life. The PCB is the control board that drives the spark, reads the flame signal back, and governs the whole ignition sequence, and a fault here tends to show up as inconsistent or repeated lockouts, sometimes alongside other unrelated fault codes appearing on the same boiler.
Replacing the electrode is generally a straightforward mechanical job, but the ignition electrode is a gas-carrying component and this work must be carried out by a Gas Safe registered engineer. PCB diagnosis and replacement carries the same restriction, since it involves working inside the gas and electrical control system of the appliance.
For background on what's actually involved in swapping a control board, see the guide on boiler PCB replacement, and for a clearer picture of where the legal line sits between what a homeowner can check themselves and what needs a registered engineer, see the Gas Safe rules for boiler repairs.
Spare parts, warranty, and the GC number
What is the GC number and why does it matter when ordering a part?
The GC number identifies the exact production variant of your boiler. Worcester Bosch has changed internal components across production runs while keeping the same model name, so a part that fits one Greenstar 8000 could be wrong for another built a year later. The GC number is printed on the data badge inside the front panel, and quoting it when you order narrows the search to the build you actually have.
Will fitting a non-genuine part affect my Worcester Bosch warranty?
It can, if the part is judged to have caused or contributed to the fault being claimed for. A compatible part correctly fitted for an unrelated issue is a different situation, but warranty terms vary by policy and registration date, so check the paperwork that came with your boiler before deciding. Keep the receipt for any part you buy and a record of who fitted it.
Can I fit a compatible part myself, or does it have to be an original Worcester Bosch part?
Plenty of components have compatible equivalents from other manufacturers, and these can work fine on the same appliance. Whether you're allowed to fit it yourself is a separate question from whether it fits, and depends on whether the component carries gas. This work must be carried out by a Gas Safe registered engineer wherever the part is part of the gas train, including the gas valve and combustion assembly, and that restriction applies whether the part is genuine or a compatible equivalent.
Where do I find the part number for a specific component?
Worcester Bosch part numbers are given in the manufacturer's service manual for each model range, and they're tied to the GC number. The existing part usually carries its own label too, which is worth checking against before you order, since a running change part way through a production run can leave two versions of the same component in circulation.
Does having a repair done by someone other than a Worcester Bosch engineer affect the warranty?
Not on its own. What matters is whether the person doing the work was competent to do it, which for anything touching the gas supply means Gas Safe registered, and whether the part used was suitable for the appliance. An unregistered person working on a gas component is a legal problem before it's a warranty one.
Find verified suppliers for Worcester Bosch Greenstar parts
Once you know which part has failed, the next step is finding someone who stocks it.