Worcester Bosch 2911 fault codewhat it means and how to fix it
Calibration failed. The Greenstar 2000 sets its own combustion by reading the flame, and the reading did not add up. Worcester check the electrode, its earth, the siphon, then the gas valve or board.
Take care
Get a Gas Safe registered engineer to look at your Worcester Bosch 2911
We do not pass enquiries to a repair firm, so there is no form here. Find a Gas Safe registered engineer through the Gas Safe Register, and ask to see their registration card when they arrive — it is the register, not the van, that tells you they are qualified.
What 2911 means
The Greenstar 2000 and Worcester 2000 differ from the 8000 Life in one important way: they have automatic combustion control, which means the boiler calibrates its own gas-air ratio by measuring the ionisation current in the flame rather than having an engineer set it with an analyser. 2911 is that calibration failing, and it comes with a family. 2912 is no flame signal during calibration, 2913 flame signal too low during calibration, 2917 no flame signal when checking the combustion control, and 2918 a fault in combustion control whose remedy is to clean the siphon and drain water from the gas side.
Worcester's checks for 2911 and 2912 are the same list they use for flame-sensing faults: the tip about the ceramic insulator on the electrode, which can crack and let the spark go to earth — twist and pull it to be sure it is not broken — then the flame sense electrode and its lead, the earth tags on the boiler and the electrodes, the siphon and any water collecting in the sump, and finally the air-gas ratio valve and the control or burner control unit. For 2913 the remedy is simply the flame sense electrodes and their leads.
For 2917 Worcester give a procedure worth knowing: disconnect the power and reconnect it, trigger a heat demand, wait five minutes, and if the fault recurs within that time reset the appliance without disconnecting the supply, which triggers a fresh calibration of the ionisation circuits. Because the whole system rests on a clean electrode reading a clean flame, these codes are the 2000's way of saying the flame signal is not trustworthy, and they arrive most often after a service, a component change or a period of condensate backing up into the sump.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| Flame sense electrode or lead | The heart of the calibration. Worcester's tip is the ceramic insulator, which cracks and lets the spark earth out; then the electrode itself and its lead. | Common |
| Earth tags on boiler or electrodes | The ionisation current needs a good return path. Worcester list the earth tags after the electrode. | Common |
| Water in the sump, siphon not draining | On Worcester's list for 2911 and 2912, and the whole remedy for 2918. Condensate backing up disturbs the flame and the reading. | Occasional |
| Gas-air ratio valve or control unit | Worcester's last resort on both lists: replace the air and/or gas ratio control valve, then the control unit or burner control unit. | Rare |
Can you fix it yourself?
Stop — this needs an engineer
What it costs to fix
Typical repair cost
£90–£480
Time on site
1 hour
Electrode and lead £130–£220 fitted. Siphon clean is part of a service. Gas-air ratio valve £300–£480 if it comes to that.
Typical UK prices including parts and labour, updated September 2026. Your quote may differ.
Parts you may need
We may earn a commission if you buy a part through one of these links. It does not change what you pay, and it does not affect which parts we list.
Worcester Bosch Greenstar electrode set
87161431020£22–£52Authorised Worcester parts distributor
Price checked 28 September 2026
Worcester Bosch Greenstar gas valve
£180–£320Authorised Worcester parts distributor
Price checked 28 September 2026
Condensate trap and siphon
COND-TRAP£15–£38Plumbing merchant
Price checked 28 September 2026
Own this appliance and had this code? . Nothing is published until a person has read it.
Related Worcester Bosch fault codes
- EA 227Stop — call an engineerThe boiler tried to light and never detected a flame, so it has shut the gas off and locked out. This is the most common Worcester fault code.Related: ignition and gas supply
- EA 229Get it checkedThe burner lit and then the flame was lost during operation. That is a different fault from failing to light in the first place.Related: ignition and gas supply
- F7 228Stop — call an engineerA flame is being detected before the burner has started. Switch the appliance off and get a Gas Safe engineer rather than resetting.Related: ignition and gas supply
- FA 306Stop — call an engineerThe boiler detected a flame after it had closed the gas valve. That points to a valve letting gas past when it should be shut, and it must not be reset.Related: ignition and gas supply
- 2951Stop — call an engineerThe flame has been lost too many times in a row and the boiler has locked out. Each loss was a 229; Worcester's instruction is to read the history and find what put it out.Related: ignition and gas supply
- EA 261Stop — call an engineerAn ignition fault that Worcester define two ways: on the Greenstar i it points at the heat control module, on the Compact range it means the flame was not established inside the four-second ignition window.Related: ignition and gas supply
Own a Worcester Bosch boiler? Save it and its codes come first next time.
Related guides
Common questions about Worcester Bosch 2911
What does calibration mean on a boiler?
The Greenstar 2000 adjusts its own gas-air ratio by measuring the flame's ionisation current instead of relying on an engineer's analyser setting. Calibration is that self-adjustment, and 2911 means it could not complete.
Are 2911, 2912, 2913 and 2917 the same fault?
Same family, different stage. 2911 is the calibration failing, 2912 no flame during it, 2913 a flame signal too weak to use, 2917 no signal when the combustion control is checked. Worcester's checks overlap almost entirely.
Does the 8000 Life have these codes?
No. The 8000 Life, 8000 Style and 4000 have their combustion set with an analyser and their tables do not include the calibration codes.
Technical review
Not reviewed by an engineer
No engineer has read this page. Severity follows what the manufacturer publishes, and we would rather tell you that than let a byline imply a sign-off.
Sources and how we verified this
We establish the facts from the sources below and then write the explanation ourselves. We do not copy manufacturer wording and we do not host their manuals.
- Manufacturer technical and service documentation
This page has not been reviewed by an engineer, and we would rather tell you that than imply otherwise. The severity rating follows what the manufacturer publishes: where they say isolate the appliance and call someone, so do we.