Worcester Bosch Fb 365 fault codewhat it means and how to fix it
The gas valve failed its own leak test. Worcester publish Fb 365 for the first valve seat and FA 364 for the second; both mean gas may pass a valve that should be shut.
Stop — this needs an engineer
Get a Gas Safe registered engineer to look at your Worcester Bosch Fb 365
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.
Can you smell gas?
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.
Yes, near the boiler or anywhere in the house
Gas may be passing the valve into the burner compartment and out through the flue or case. Do not reset the boiler.
Turn the gas off at the meter, open the windows, put out any flame and do not operate electrical switches. Call the National Gas Emergency Service on 0800 111 999.
No smell, the boiler is just locked out
The leak the boiler found is small enough to be contained, but a gas valve that fails its own test is finished.
Leave the boiler off and book a Gas Safe registered engineer. Quote both the letters and the number.
What Fb 365 means
The Compact family manuals publish a pair of locking codes that do not appear on the earlier displays: "FA 364 — gas valve EV2 leak test failed — gas valve leak" and "Fb 365 — gas valve EV1 leak test failed — gas valve leak". A modern gas valve has two solenoid-operated shut-offs in series, EV1 and EV2, and the control tests them: it closes one, opens the other, and watches for the pressure change that a sealed valve should not produce.
A test failure means one of the two seats is letting gas through when it is meant to be closed. The boiler locks out and will not restart until reset, and Worcester specify the reset button for both codes. This is not a fault to reset and forget. The whole point of two valves is that a single leaking seat is still contained by the other, and the boiler has just told you that one half of that protection has failed. It sits alongside FA 306 — a flame detected after the valve has closed, which is the same leak found a different way, by seeing it burn — and the two codes together are why the FA family is treated as gas-side, engineer-only.
Because the valve on a Compact is a sealed unit, the repair is replacement, followed by a tightness test of the installation and a combustion check. It is not common. When it does appear it tends to be on a boiler of some age, or one where debris in the gas supply has reached the valve.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| Worn or contaminated valve seat | A seat that no longer seals lets a small flow past when the coil is de-energised. Debris carried in the gas supply, or simple age, are the usual reasons. | Common |
| Solenoid not closing fully | A coil that is weak, or a plunger that sticks, leaves the valve fractionally open. Worcester's coil resistance checks find an electrical cause; a mechanical one is found by the test itself. | Occasional |
| Pressure sensing fault giving a false failure | The test relies on the control reading pressure correctly. A fault in that measurement can report a leak that is not there, which is one reason the engineer confirms the leak with a manometer before condemning the valve. | Rare |
Can you fix it yourself?
Stop — this needs an engineer
What it costs to fix
Typical repair cost
£300–£480
Time on site
1h 30m
Gas valve replacement £300–£480 fitted, including the tightness test and combustion check.
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 gas valve
£180–£320Authorised Worcester parts distributor
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 Fb 365
What is the difference between 364 and 365?
Which of the two valve seats failed. Worcester label them EV1 and EV2, and publish Fb 365 for EV1 and FA 364 for EV2. To you they mean the same thing: the gas valve is not sealing and has to be replaced.
How is this different from FA 306?
FA 306 is a flame detected after the valve has shut, which is the same underlying leak caught by the flame sensor rather than by the pressure test. The Compact range runs the leak test on every cycle so it usually catches the fault first.
Can I reset it once, as with other codes?
Worcester specify the reset button for both codes, so a single reset is permitted. If there is any smell of gas, do not — turn off at the meter and call 0800 111 999. If it resets and runs, still get it checked; the test failed for a reason.
What will it cost?
A gas valve on a Greenstar Compact is £300 to £480 fitted, including the tightness test and combustion check that follow any gas-side work.
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
- 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.