Worcester Bosch FB Fault Code: Causes, Fixes & Repair Costs
What does the Worcester Bosch FB fault code mean?
The FB fault code (most commonly displayed as FB 365) means the boiler's internal safety system has run a leak test on gas valve solenoid EV1 and the valve has not passed. When this test fails, the boiler locks out completely and will not fire again until a manual reset is performed — but a reset alone will not fix the underlying problem. The boiler is essentially refusing to operate because it cannot confirm the gas valve is holding pressure safely. This is a locking fault, meaning a flashing code and warning triangle will appear on the display. Note: the closely related FA fault code (FA 364) covers solenoid EV2 on the same valve body — if you are seeing both FA and FB, both solenoids may be involved.
General guidance only — not a substitute for professional advice, and not certified by a Gas Safe engineer. Always have your specific appliance assessed by a qualified professional. Any gas work must be carried out by a Gas Safe registered engineer. If you smell gas or suspect carbon monoxide, leave the property and call the National Gas Emergency line on 0800 111 999.
Common causes
- Faulty or worn gas valve (EV1 solenoid) Common
Over years of use, the EV1 solenoid inside the modulating gas valve can wear, corrode, or develop internal faults that prevent it from sealing correctly during the boiler's self-test cycle. This is the most frequent reason the FB code appears and almost always requires the valve to be replaced.
- Gas valve not holding pressure during leak test Common
Even a valve that is mechanically intact can fail the internal leak test if there is a minor internal seal failure or debris preventing a proper gas-tight shut-off. The boiler registers this as a safety-critical failure and locks out immediately.
- Ignition lead or electrode fault Sometimes
A damaged ignition lead, cracked electrode, or poor earth connection can produce erratic flame signals that the boiler's burner control unit misinterprets as a valve-related anomaly. Inspecting and replacing ignition components is one of the first checks a Gas Safe engineer will carry out.
- PCB (printed circuit board) misreading the valve signal Sometimes
If the gas valve itself tests within specification, the control board may be sending an incorrect drive signal to EV1 or misreading the feedback from it. PCB faults are less common but should be considered after the valve and ignition components have been ruled out.
- Combustion or flue gas readings outside limits Rare
Incorrect CO₂ levels in the flue exhaust — caused by a partial blockage, a flue gas recirculation problem, or incorrect gas rate — can affect combustion-related readings and trigger valve fault codes as the boiler tries to protect itself.
How to fix it
- Do not repeatedly reset the boiler DIY safe
A single manual reset is reasonable to attempt, but if the boiler locks out again immediately or within a short time, stop resetting. Repeatedly forcing the boiler to attempt ignition when a gas valve fault is present can cause further damage and is unsafe.
- Check for a gas smell and confirm your gas supply is working DIY safe
Check that other gas appliances (cooker, gas fire) are working normally to rule out a wider supply interruption. If you can smell gas anywhere in or around your home, do not touch the boiler — leave the property, avoid using electrical switches, and call the National Gas Emergency Service on 0800 111 999.
- Note the full fault code displayed DIY safe
The display should show the code alongside a numeric sub-code — for this fault it will typically read FB 365. Write down exactly what you see, including any secondary digits, as this helps the engineer narrow down the fault before arriving.
- Contact a Gas Safe registered engineer to diagnose the valve Gas Safe engineer
A qualified engineer will test EV1 using combustion analysis equipment and specialist valve-testing tools, check the gas rate and pressure at the valve, and assess whether the valve body needs replacing. This work must only be carried out by a Gas Safe registered engineer — it is illegal and dangerous for anyone else to work on gas components.
- Engineer to inspect ignition leads, electrodes, and burner components Gas Safe engineer
If the valve tests within acceptable limits, the engineer will inspect the ignition leads for cracking or tracking marks, check electrode gaps and condition, and test the burner control unit to rule out a false trigger of the FB code.
- Engineer to test or replace the gas valve if confirmed faulty Gas Safe engineer
A confirmed faulty EV1 solenoid or gas valve body will need to be replaced with a manufacturer-approved part. The engineer must also carry out a full combustion analysis and gas-tightness test after fitting any new valve before returning the boiler to service.
- Engineer to assess the PCB if the valve tests satisfactory Gas Safe engineer
If every mechanical component checks out, the control board becomes the likely culprit. The engineer may be able to verify this with live voltage and signal testing across the PCB terminals before committing to a replacement.
- Allow the engineer to reset and commission the boiler Gas Safe engineer
Once the root cause has been resolved, the engineer will reset the fault, run a full commissioning check, and confirm safe operation before leaving. Do not attempt to reset the boiler yourself after a Gas Safe engineer has begun work unless they specifically ask you to.
Parts you may need
- Modulating gas valve (EV1/EV2 assembly) · from £120
- Ignition lead set · from £25
- Ignition electrode · from £20
- Burner control unit / ionisation board · from £85
- PCB (main printed circuit board) · from £175
The exact spare depends on your boiler's GC number (on the data badge). Check this against the part before buying.
Typical repair cost
Expect to pay roughly £150–£320, depending on the underlying cause.
Frequently asked questions
Is the FB fault code dangerous — should I keep using my boiler?
Because the FB code specifically relates to a gas valve leak test failure, you should not attempt to keep resetting and running the boiler. The boiler has locked out as a safety precaution. There is no immediate danger from simply having a locked-out boiler, but if you can smell gas at any point you must stop, leave the property, and call 0800 111 999 immediately. Call a Gas Safe engineer as soon as possible to inspect it properly.
What is the difference between the FA and FB fault codes on a Worcester Bosch boiler?
Both codes point to the same modulating gas valve but to different solenoids within it. FA (sub-code 364) relates to solenoid EV2 failing its leak test, while FB (sub-code 365) relates to solenoid EV1. If you see both codes appearing, it suggests the entire gas valve body needs replacing rather than just one component within it.
How much does it cost to fix the FB fault code in the UK?
For most homeowners, a gas valve replacement including parts and labour from a Gas Safe engineer runs between £150 and £320. If only the ignition leads or electrodes are at fault, the cost is considerably lower — typically £80 to £150 all in. A PCB replacement, which is sometimes needed if the valve itself is fine, can cost £250 to £450 depending on the boiler model. These are typical figures for 2025; actual quotes will vary by region and engineer.
My Worcester Bosch boiler keeps showing FB after being reset — what should I do?
If the boiler locks out with the FB code repeatedly, it is a strong sign the underlying fault has not been resolved. Repeated resetting without a repair does not fix the gas valve and can mask deteriorating conditions. Stop resetting, note the full code on the display (it should say FB 365), and arrange for a Gas Safe registered engineer to carry out a proper diagnosis. If your boiler is over 10 years old and the gas valve has failed, it is also worth getting a quote for a replacement boiler to compare against repair costs.