Worcester Bosch FA 364 Fault Code: Causes, Fixes & Repair Costs
What does the Worcester Bosch FA 364 fault code mean?
The FA 364 code is a locking fault that appears when the boiler's internal self-diagnostic test detects that the EV2 gas valve has failed its leak-integrity check. The boiler suspects the EV2 valve is allowing gas to pass through even when it should be closed — detected because a flame signal is sensed after the burner has been commanded to shut off. When this happens, the boiler locks out completely, the FA code flashes on the display alongside a warning triangle symbol, and the fault indicator LED also flashes. The boiler will not restart until the fault is cleared and a manual reset is performed. Note the three-digit suffix matters here: 364 specifically identifies the EV2 valve leak test, whereas the related code FB 365 refers to the same test failure on the EV1 valve. Do not repeatedly reset this fault — if you smell gas at any point, leave the property immediately, avoid operating any electrical switches, and call the National Gas Emergency Service on 0800 111 999 from outside.
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 leaking EV2 gas valve Common
The EV2 valve is a safety-critical component of the modulating gas valve assembly. Over time the internal seals and mechanical parts wear, which can cause the valve to fail to close fully, allowing a small amount of gas to pass through when the boiler is off. The boiler's self-test detects this residual flame signal and triggers the lockout. This is the most frequent root cause of FA 364.
- Loose or corroded wiring to the gas valve Sometimes
Vibration during normal boiler operation can gradually loosen the electrical connectors on the gas valve harness. Corroded or intermittent connections produce incorrect voltage signals, causing the PCB to misinterpret the valve's state — reporting it as open when it should be closed or vice versa. A Gas Safe engineer will test the wiring with a multimeter and reseat or replace any suspect connectors.
- Ionisation or flame-detection sensor fault Sometimes
The ionisation electrode monitors whether a flame is present. If the electrode is contaminated, cracked, or its cable has shorted to earth, the PCB may register a phantom flame after shutdown — mimicking the same condition as a leaking EV2 valve and triggering FA 364. Inspecting and cleaning or replacing the ionisation probe and its lead can resolve this.
- Faulty PCB (printed circuit board) Rare
The PCB controls the gas valve relay and interprets all sensor inputs. A PCB fault can cause it to misread the valve's status or incorrectly fail the internal leak test even when the valve itself is fine. PCB faults are usually diagnosed only after the gas valve, wiring, and ionisation circuit have been ruled out, as PCBs are expensive and misdiagnosis is common.
How to fix it
- Do not operate the boiler if you smell gas DIY safe
If there is any smell of gas, do not touch the boiler or any electrical switches. Open windows if safe to do so, leave the property, and call the National Gas Emergency Service on 0800 111 999 from outside. Only return when told it is safe to do so.
- Check your gas supply is working DIY safe
Before calling an engineer, confirm that other gas appliances in your home (such as a gas hob) are working normally. If nothing gas-powered is working, contact your gas supplier — the issue may be an interrupted supply rather than a boiler fault.
- Attempt a single reset — once only DIY safe
If there is no smell of gas and other appliances are working, you may try pressing the reset button once as directed in your boiler manual. If the boiler locks out again, do not reset it a second time. FA 364 is a safety lockout; repeated resetting without fixing the underlying fault risks damaging the boiler further and is unsafe.
- Gas Safe engineer: inspect the EV2 gas valve and wiring harness Gas Safe engineer
A qualified engineer should check the gas valve assembly for signs of wear, internal leakage, or mechanical seizure. The wiring connectors and harness continuity should be tested with a multimeter. Loose or corroded connections should be repaired or replaced before condemning the valve itself.
- Gas Safe engineer: inspect the ionisation electrode and cable Gas Safe engineer
The engineer should examine the ionisation probe for carbon deposits, cracks, or a damaged insulation sleeve on the cable. A shorted ionisation circuit can simulate the leak-test failure. Cleaning or replacing the electrode and lead is straightforward and relatively inexpensive.
- Gas Safe engineer: replace the EV2 gas valve if required Gas Safe engineer
If the valve is confirmed as faulty or leaking, it must be replaced. The engineer will source either an OEM Worcester Bosch replacement or a compatible approved equivalent, fit it, and carry out combustion analysis and a full leak test before returning the boiler to service.
- Gas Safe engineer: test or replace the PCB if all other components are sound Gas Safe engineer
If the gas valve, wiring, and ionisation components all test correctly but the fault persists, the PCB may be misreading the leak-test result. PCB replacement is a last resort and should only be carried out after all other causes are eliminated, as it is a costly part.
- Consider boiler replacement if multiple major components are failing DIY safe
If your boiler is over 10–12 years old and the engineer identifies several worn components alongside the valve fault, it may be more economical to invest in a new boiler rather than spending on successive repairs. Ask your engineer for an honest assessment before authorising expensive parts.
Parts you may need
- Modulating gas valve assembly (EV2) · from £160
- Ionisation electrode · from £18
- Ionisation electrode cable / lead · from £22
- Gas valve wiring harness · from £35
- PCB (printed circuit board) · from £280
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–£350, depending on the underlying cause.
Frequently asked questions
Is FA 364 dangerous — should I be worried about a gas leak?
You should take this fault seriously. FA 364 means the boiler's own safety test suspects the EV2 valve is not sealing correctly, which could indicate gas is passing through when it shouldn't be. If you smell gas at any point, treat it as a gas emergency: leave the property and call 0800 111 999. If there is no smell of gas, the immediate risk is lower, but you should still stop using the boiler and call a Gas Safe engineer promptly — do not keep resetting the lockout.
What is the difference between FA 364 and FB 365?
Both codes relate to the boiler's internal gas valve leak test, but they identify different valves. FA 364 means the EV2 valve has failed its leak-integrity check, while FB 365 means the same failure has been detected on the EV1 valve. The diagnosis and repair approach is very similar for both, but the specific valve being replaced will differ. Make sure you quote the full code including the three-digit suffix to your engineer.
How much does it cost to fix a Worcester Bosch FA 364 fault in the UK?
For most homeowners the repair involves replacing or reconditioning the EV2 gas valve, which typically costs £150–£350 including parts and labour. If the fault turns out to be something simpler — such as a loose connector or a faulty ionisation lead — the bill could be under £150. In rare cases where the PCB is also faulty, costs can rise above £400. If your boiler is older and several components need attention, your engineer may recommend a full boiler replacement instead, which for a like-for-like combi swap typically costs £1,400–£2,000 installed.
Can I reset the boiler myself or do I need an engineer?
You may try a single reset if there is no smell of gas, following the instructions in your boiler manual. However, FA 364 is a locking safety fault — if the boiler locks out again after one reset, stop and call a Gas Safe registered engineer. Repeatedly resetting a gas valve lockout without fixing the root cause is unsafe and can cause further damage to the boiler.