Worcester Bosch FA 306 Fault Code: Causes, Fixes & Repair Costs
What does the Worcester Bosch FA 306 fault code mean?
The FA 306 fault code indicates that your Worcester Bosch boiler has detected an ionisation current — in other words, it believes a flame is present — after the burner has already shut down. Because the boiler cannot confirm whether this is a genuine flame or a sensor error, it treats the situation as a potential safety hazard and locks out completely. You will see the FA 306 code displayed alongside a flashing triangle symbol, and the boiler will not restart until you manually reset it. This is classed as a locking fault, meaning a simple power cycle is not enough — you must press and hold the reset button on the control panel. If the code returns immediately or repeatedly, professional diagnosis is required.
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
- Moisture or contamination on the electrode assembly Common
Water or condensation on the ionisation electrode is one of the most frequently seen triggers for this code. Even a small amount of moisture can create a residual current that the boiler interprets as a flame still burning. This is particularly common after a period of damp weather or if the boiler has been sitting idle.
- Damaged or degraded ionisation cable Common
The ionisation cable carries the tiny electrical signal from the electrode to the PCB. If this cable has become frayed, cracked, or has a loose connection at either end, it can produce a rogue signal that mimics a live flame. Worcester Bosch recommends inspecting electrodes and associated wiring as part of every major service.
- Faulty or sticking gas valve Sometimes
The gas valve is responsible for cutting off the gas supply the moment the burner shuts down. If the valve is worn or sticking and allows a small amount of gas to continue flowing, combustion can persist briefly after shutdown — generating a genuine ionisation signal at exactly the wrong moment.
- Faulty air-to-gas ratio control valve Sometimes
This valve governs the balance between air and gas entering the burner. When it malfunctions, combustion can become unstable, making the flame behaviour unpredictable. This can confuse the ionisation detection circuit and cause the boiler to log a false flame event during shutdown.
- PCB fault Rare
The printed circuit board interprets the signal from the ionisation electrode and controls the gas valve accordingly. A PCB that is failing may misread the ionisation signal, fail to close the gas valve at the right moment, or generate a spurious lockout. PCB faults are less common but should be considered if other components test normal.
How to fix it
- Reset the boiler once DIY safe
Locate the reset button on your Worcester Bosch control panel — it is usually marked with a flame or reset symbol. Press and hold it for around three seconds until the boiler attempts to restart. One reset attempt is a reasonable first step. If the FA 306 code clears and the boiler runs normally, monitor it over the next hour or so.
- Do not reset the boiler more than two or three times DIY safe
If the code returns immediately after reset, or comes back within a short time, stop resetting. Repeated resets on a false flame fault can mask a genuine safety issue. If you can smell gas at any point, do not reset the boiler — turn off the gas supply at the meter, open windows, leave the property, and call the National Gas Emergency Service on 0800 111 999.
- Check for any obvious signs of moisture around the boiler DIY safe
Have a visual look around the boiler casing for signs of condensation, dripping water, or damp. Do not open the boiler casing yourself. If you can see water pooling near the boiler or its pipework, note this down to tell the engineer — it may point to a condensate issue or a slow internal leak contributing to electrode moisture.
- Book a Gas Safe registered engineer for diagnosis Gas Safe engineer
Because FA 306 involves the flame detection and gas valve system, all further investigation must be carried out by a Gas Safe registered engineer. They will inspect the ionisation electrode and cable for damage or moisture, test the gas valve for correct closure, check the air-to-gas ratio control valve, and assess the PCB if all other components test satisfactory.
- Engineer to inspect and clean or replace the electrode assembly Gas Safe engineer
The engineer will check the ionisation electrode for cracking, heavy carbon deposits, or moisture ingress. If the electrode is simply dirty or damp, cleaning may be sufficient. Worcester Bosch advises replacing electrodes approximately every five years as a preventative measure, and an engineer may use wire wool to clean them if they are not yet worn through.
- Engineer to test and replace the gas valve if necessary Gas Safe engineer
If the electrode and cable are in good condition, the engineer will test whether the gas valve is seating and closing correctly. A valve that continues to pass gas after shutdown requires replacement — this is a gas-carrying component and must only be handled by a qualified Gas Safe engineer.
- Engineer to assess the PCB if all other components are serviceable Gas Safe engineer
If the electrode, ionisation cable, and gas valve all test within specification, the PCB may be generating or misreading the ionisation signal. The engineer will test the control board and advise on repair or replacement.
Parts you may need
- Ionisation electrode · from £55
- Ionisation / ignition cable set · from £40
- Gas valve · from £120
- Air-to-gas ratio control valve · from £90
- 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 £150–£380, depending on the underlying cause.
Frequently asked questions
Is FA 306 dangerous — should I leave my house?
FA 306 on its own is a safety lockout rather than confirmation of a gas leak, but you should treat it seriously. The boiler has detected something abnormal with its flame circuit and has shut itself down as a precaution. Do not keep resetting it. If you can smell gas at any point, do not touch the boiler — turn off the gas at the meter, ventilate the property, and call the National Gas Emergency Service on 0800 111 999 immediately. If there is no smell of gas, the property is safe to remain in, but you should arrange for a Gas Safe engineer to inspect the boiler promptly.
Can I fix FA 306 myself?
There is one homeowner-safe step: a single reset attempt using the button on the control panel. Beyond that, FA 306 involves the gas valve, ionisation circuitry, and potentially the PCB — all of which must only be worked on by a Gas Safe registered engineer. Opening the boiler casing yourself is not safe, and any work on gas-carrying components without Gas Safe registration is illegal under UK law.
How much does it cost to fix a Worcester Bosch FA 306 fault?
Most people with this fault pay somewhere between £150 and £380 all in, depending on which component is at fault. Electrode replacement is towards the lower end at around £100–£150 including labour. Gas valve replacement typically costs £180–£250 once parts and labour are included. In the relatively uncommon situation where the PCB needs replacing, costs can rise to £400–£600 or more, so if your boiler is over 10–12 years old and the PCB has failed, it is worth comparing that cost against a new installation.
Why does my Worcester Bosch keep showing FA 306 after I reset it?
If the code returns quickly or persistently after reset, the underlying fault has not resolved itself. Common reasons include a moisture problem on the electrode that has not dried out, a gas valve that is not sealing correctly, or a failing PCB. Continuing to reset the boiler without fixing the root cause will not help and could mask a genuine safety issue. Arrange a visit from a Gas Safe engineer so the fault can be properly diagnosed and repaired.