Worcester Bosch FA Fault Code: Causes, Fixes & Repair Costs
What does the Worcester Bosch FA fault code mean?
The FA fault code means your Worcester Bosch boiler has detected a flame signal at a point when the gas valve should be fully closed and no combustion should be taking place. In plain terms, the boiler's ionisation circuit is picking up what it believes to be an active flame after the burner has been instructed to stop — a situation the boiler treats as a potential safety hazard and responds to by locking out. Two sub-codes are commonly displayed alongside FA: FA 306 indicates a persistent false flame signal after the burner has shut down, while FA 364 points to a leak-test failure on gas valve solenoid EV2 (a closely related code, FB 365, flags the same failure on solenoid EV1). All three variants share the same underlying concern — unintended gas or flame activity — and should be treated with the same level of urgency. This fault is the opposite of the EA code (no flame detected during ignition); FA is arguably the more serious of the two because it suggests gas may be passing through the valve when it should not be.
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 gas valve Common
The gas valve contains two solenoids that must seal completely when the burner shuts down. If either solenoid is worn, sticking, or has degraded seals, a small amount of gas can continue to pass through, sustaining a residual flame or triggering the ionisation circuit. FA 364 and FB 365 sub-codes specifically point to the boiler's own internal gas valve leak test detecting this condition.
- Moisture or contamination on the ionisation electrode Common
The ionisation electrode works by passing a tiny electrical current through the flame; the conductivity of the flame completes a circuit. If moisture, combustion deposits, or corrosion bridge the electrode tip to earth, the PCB can receive a spurious signal that mimics a flame even when none is present. This can cause an FA lockout without any genuine gas valve problem.
- Damaged or degraded ionisation electrode Sometimes
A cracked ceramic insulator or a badly corroded electrode tip can produce erratic electrical readings. Rather than intermittently shorting as with moisture, physical damage tends to generate a more persistent false flame signal, making the fault harder to clear with a single reset.
- Faulty or damaged ionisation cable Sometimes
The cable connecting the electrode to the PCB carries a very low-level signal. If the cable's insulation is cracked, if it has been pinched during a previous service, or if the connector has corroded, it can introduce stray electrical current that the PCB interprets as a flame signal.
- PCB fault Rare
The PCB processes the signal from the ionisation electrode and controls the gas valve. If the relevant input circuit on the PCB has failed, it may report a flame signal regardless of what the electrode is actually detecting. PCB faults tend to appear after the other causes have been ruled out, or alongside other unrelated error codes.
How to fix it
- Check for a gas smell and act immediately if present DIY safe
Before doing anything else, check whether you can smell gas near the boiler or elsewhere in the property. If you can, do not touch the boiler, do not operate any electrical switches, leave the building, and call the National Gas Emergency Service on 0800 111 999. Only proceed with the steps below if there is no smell of gas.
- Attempt a single boiler reset DIY safe
Locate the reset button on your boiler's control panel — on most Greenstar models this is a dedicated button or a flame symbol on the display. Press and hold it for around 3–5 seconds until the boiler attempts to restart. If the FA code clears and the boiler runs normally, monitor it closely over the next hour. Do not reset the boiler more than once or twice; repeated resets on an FA fault can mask a genuine safety issue and should be avoided.
- Note the full sub-code shown on the display DIY safe
Write down the complete code your boiler is showing — for example FA 306, FA 364, or FB 365. This information will help a Gas Safe engineer diagnose the fault significantly faster. FA 364 and FB 365 in particular indicate the boiler's internal gas valve leak test has failed, which will need prompt attention.
- Do not attempt internal inspection or component access Gas Safe engineer
The gas valve, ionisation electrode, ionisation cable, and PCB are all internal components that require removal of the boiler casing to access. Opening the casing and working on these parts without Gas Safe registration is illegal for gas-related components, dangerous, and will invalidate your boiler warranty. Leave all internal inspection to a qualified engineer.
- Contact a Gas Safe registered engineer to diagnose and repair the fault Gas Safe engineer
An engineer will carry out a structured diagnosis: testing the gas valve solenoids and seals for leakage, inspecting and cleaning or replacing the ionisation electrode and cable, and checking PCB inputs. If the gas valve is found to be passing gas when closed, it will need to be replaced — this is not a repair that can be deferred. Always ask to see the engineer's Gas Safe ID card before work begins.
Parts you may need
- Ionisation electrode · from £35
- Ionisation / detection cable · from £25
- Gas valve (replacement unit) · from £180
- PCB (printed circuit board) · from £220
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 £120–£380, depending on the underlying cause.
Frequently asked questions
Is the FA fault code dangerous — do I need to turn the boiler off?
The FA code indicates the boiler has detected a flame when it should not have, which is a genuine safety concern. The boiler locks itself out automatically as a precaution, so it will not continue to fire. You do not need to isolate the boiler manually unless you can smell gas — if you do smell gas, turn off the gas supply at the meter, do not use any electrical switches, and call 0800 111 999. In the absence of a gas smell, the lockout itself is the boiler doing its job; arrange for a Gas Safe engineer to visit promptly rather than treating it as an emergency requiring evacuation.
What is the difference between FA 306 and FA 364 on a Worcester Bosch boiler?
FA 306 means the boiler detected a persistent flame signal after the burner shut down — this most commonly points to a faulty ionisation electrode, moisture on the electrode, a wiring fault, or a gas valve that is not sealing fully. FA 364 is more specific: it means the boiler ran its own internal gas valve leak test (on the EV2 solenoid) and that test failed, strongly suggesting the valve itself is passing gas when it should be closed. The closely related code FB 365 is the same test but for the EV1 solenoid. If you are seeing 364 or 365, a gas valve replacement is the most likely outcome.
Can I keep resetting the boiler to get hot water while I wait for an engineer?
We would strongly advise against it. Unlike some minor faults where a reset or two is a reasonable temporary measure, the FA code relates to potential uncontrolled gas flow or a failed safety test. Repeated resets override the boiler's own protective lockout and could allow an unsafe condition to persist. Book an engineer as soon as possible and avoid resetting more than once.
How much does it typically cost to fix an FA fault code in the UK?
Most homeowners with an FA fault pay somewhere between £120 and £380 all-in, depending on the root cause. An electrode clean or replacement at the lower end of that range is the cheapest outcome; a gas valve replacement, including parts and labour, typically lands in the £250–£380 range. PCB replacement is less common for this specific fault but can cost considerably more — often £350–£500 with labour — and on older boilers it may be worth comparing that against the cost of a new boiler installation.