This fault can involve incomplete combustion and carbon monoxide.
If anyone in the property feels sick, dizzy or drowsy, or you can smell gas: open the windows, leave the building, and do not switch anything on or off. Then call the National Gas Emergency Service.
0800 111 999Worcester Bosch FA 306 fault codewhat it means and how to fix it
The 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.
Stop — this needs an engineer
Get a Gas Safe registered engineer to look at your Worcester Bosch FA 306
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 can you smell or hear?
The right answer depends on what you can see. Pick the one that matches.
Choose the option on the left that matches what your appliance is doing, and the answer for that case appears here.
You can smell gas
Treat this as an emergency and stop reading. Do not touch the boiler, any switch, or any electrical appliance.
Open windows, get everyone out, and ring the National Gas Emergency Service on 0800 111 999 from outside the property.
A carbon monoxide alarm has sounded
Combustion is happening somewhere it should not be, or products are not leaving as they should. This needs immediate action, not diagnosis.
Get everyone into fresh air, ring 0800 111 999, and seek medical advice for anyone with headache, nausea or drowsiness.
Nothing obvious — it simply locked out
Most likely the electrode or a moisture path rather than the valve, but that is a conclusion only a combustion test can reach.
Leave the boiler switched off and book a Gas Safe registered engineer. Do not reset it while you wait.
What FA 306 means
FA 306 means an ionisation current was still being measured after the burner was supposed to have stopped. In plain terms the boiler shut the gas off, looked to confirm the flame had gone out, and found it had not. There are only two explanations and both are serious. Either the gas valve is passing gas when commanded closed, or the flame detection circuit is reporting a flame that no longer exists.
The boiler cannot tell which, so it treats the situation as the more dangerous of the two and locks out hard. This is the one Worcester code where the usual advice to try a reset does not apply. If gas really is getting through a closed valve, resetting invites it to burn outside a controlled cycle, and unburnt gas or products of combustion in the wrong place is how carbon monoxide incidents begin.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| Gas valve passing when closed | A worn seat or debris on it lets gas seep through after the valve has been commanded shut. | Common |
| Flame detection circuit reporting falsely | A wet, cracked or heavily carboned electrode can generate a signal that mimics a flame still burning. | Common |
| Moisture bridging the sensing electrode | Condensate finding its way onto the electrode or its lead creates a conductive path the board reads as ionisation. | Occasional |
| Burner control unit misreading the signal | Established only after the valve has been tested properly, because the safe assumption has to be the valve. | Rare |
Can you fix it yourself?
Stop — this needs an engineer
What it costs to fix
Typical repair cost
£140–£500
Electrode replacement with combustion check £140–£230. Gas valve £300–£500 fitted. Both include the safety checks that must follow work on the gas carrying components.
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
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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
- 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
- EA 234Get it checkedThe control board cannot see the gas valve coil, so it will not attempt to light. Worcester call it a gas valve error, coil disconnected. No gas is released, but it is a gas job.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 FA 306
Can I reset it just once?
No. On almost every other code one reset is reasonable. Here the fault the boiler is reporting is gas escaping past a valve that should be closed, and a reset asks the appliance to run again on that assumption. Leave it locked out.
Should I turn the gas off at the meter?
If you can smell gas, leave the property and ring 0800 111 999 rather than doing anything else. If you cannot smell gas, turning the appliance isolation valve off is a sensible precaution and worth mentioning to the engineer when they arrive.
How is this different from F7?
F7 228 is a flame signal seen before the burner starts. FA 306 is a flame signal seen after it stops. The sequence position is what matters: a signal after shutdown implicates the valve holding gas back, which is the more serious of the two.
What will it cost to put right?
An electrode and a combustion check comes to £140 to £230. A gas valve is £300 to £500 fitted. On an older appliance an engineer may reasonably advise you to weigh that against replacement.
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
- Manufacturer support portal
- Trade body guidance
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.