Worcester Bosch FB 365 Fault Code: Causes, Fixes & Repair Costs
What does the Worcester Bosch FB 365 fault code mean?
The FB 365 code appears when your Worcester Bosch boiler's internal self-diagnostic routine has run a leak test on the EV1 solenoid valve — the first of two solenoids inside the gas valve assembly — and detected that it is not sealing correctly. The boiler immediately locks out to prevent any unsafe gas passage. This is a critical safety fault: the boiler will not restart until the underlying problem has been identified and resolved by a Gas Safe registered engineer. For context, the closely related code FA 364 covers the same leak-test failure but on the EV2 solenoid; if you see FA 364 on your display, the diagnosis and remedy are very similar but the specific solenoid involved differs.
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
- Worn or deteriorated EV1 solenoid valve seat Common
Over time the internal seating surfaces of the EV1 solenoid can wear, warp, or accumulate debris, preventing a gas-tight seal when the valve closes. This is the most direct cause of a failed EV1 leak test and typically means the gas valve assembly needs replacing.
- Faulty gas valve coil or solenoid winding Common
The electromagnetic coil that drives the EV1 solenoid can develop an open circuit, short circuit, or high-resistance fault. This can cause the solenoid to behave unpredictably during the self-test, triggering a false or genuine leak-test failure.
- PCB or gas valve relay fault Sometimes
The burner control unit (PCB) governs the leak-test sequence and interprets the pressure feedback. A failing relay on the PCB or corrupted control logic can produce an FB 365 lockout even when the physical valve is in reasonable condition, making accurate diagnosis by an engineer essential.
- Ionisation signal anomaly Sometimes
An unexpected ionisation current detected either before the burner fires or after it shuts down can be grouped with FB-series gas valve faults. A contaminated or damaged ionisation probe, or a partially earthed flame sensor lead, can confuse the control sequence and result in this code appearing alongside combustion-related concerns.
- Wiring harness damage or loose connector Rare
Vibration, heat cycling, and age can cause the wiring loom between the PCB and the gas valve to develop intermittent connections or insulation breaks. A poor connection on the EV1 circuit can produce a sporadic leak-test failure without any physical problem with the valve itself.
How to fix it
- Do not attempt to reset or operate the boiler DIY safe
FB 365 is a critical gas-valve lockout. Repeatedly pressing reset when a gas valve fault is present can mask a genuine leak and risks damaging other components. Leave the boiler locked out.
- Check whether you can smell gas DIY safe
If you detect any smell of gas in or around your property, do not touch any electrical switches. Ventilate the area if it is immediately safe to do so, leave the building, and call the National Gas Emergency Service on 0800 111 999 from outside. Do not re-enter until cleared by an emergency engineer.
- Verify your gas supply is reaching other appliances DIY safe
Check that your gas hob, gas fire, or other gas appliances are working normally. If none of them operate, the issue may be with your gas supply rather than the boiler itself — contact your gas supplier.
- Note any additional codes or symptoms DIY safe
If your boiler display is cycling through more than one fault code (for example, also showing FA 364), make a note of all codes displayed. This information will help the engineer narrow down the diagnosis faster.
- Call a Gas Safe registered engineer to carry out a full gas valve diagnosis Gas Safe engineer
A qualified engineer will run pressure and combustion tests, check the EV1 and EV2 solenoids, inspect wiring, and test the PCB relay to establish whether the valve itself has failed or whether a secondary component is triggering the fault.
- Gas valve replacement and combustion analysis Gas Safe engineer
If the EV1 solenoid or full gas valve assembly is confirmed faulty, the engineer will replace the component using a manufacturer-approved part. A full combustion analysis must be completed afterwards to verify safe and efficient burner operation before the boiler is returned to service.
- Engineer-supervised reset and function test Gas Safe engineer
Once all remedial work is complete and confirmed safe, the engineer will reset the boiler, clear the fault code, and run a full operational test including hot water and heating demand to ensure stable operation.
Parts you may need
- Worcester Bosch gas valve assembly (full replacement) · from £185
- Gas valve solenoid coil (EV1 or EV2) · from £45
- Ionisation / flame-sense electrode · from £25
- PCB gas valve relay · from £35
- Wiring loom / harness (gas valve to PCB) · from £40
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 £250–£500, depending on the underlying cause.
Frequently asked questions
Can I reset my Worcester Bosch boiler when it shows FB 365?
It is best not to. Unlike a simple lockout caused by low pressure or a frozen condensate pipe, FB 365 indicates the boiler's own safety check has found a potential gas valve leak. Resetting without fixing the root cause will not clear the fault permanently and could mask a genuine safety issue. Leave the boiler locked out and call a Gas Safe engineer.
What is the difference between FA 364 and FB 365 on a Worcester Bosch boiler?
Both codes relate to a failed internal gas valve leak test, but they target different solenoids within the same valve assembly. FA 364 means the EV2 solenoid has failed its leak test, while FB 365 means the EV1 solenoid is the one at fault. In practice the diagnosis and repair approach is very similar for both — a Gas Safe engineer will test the full valve assembly and replace whichever component has failed.
How much does it cost to replace a gas valve on a Worcester Bosch boiler in the UK?
For most Worcester Bosch models, the total cost including parts and labour typically falls between £250 and £500. The gas valve assembly itself usually costs around £150–£200, with the remainder covering the engineer's time. Labour rates are higher in London and the South East. In rare cases where the PCB also needs replacing, the total can exceed £600 — but this is not the norm for a straightforward FB 365 diagnosis.
Is my boiler worth repairing if it shows FB 365?
If your boiler is under 10 years old and otherwise in good health, replacing the gas valve is usually worthwhile and significantly cheaper than a new boiler. If the boiler is over 12 years old, has a history of repeated faults, or the repair quote approaches £500 or more, it is sensible to get a replacement boiler quote at the same time. A new Worcester Bosch combi installation typically costs between £2,500 and £4,000, but it comes with a full manufacturer warranty and much better energy efficiency.