Worcester Bosch F7 328 Fault Code: Causes, Fixes & Repair Costs
What does the Worcester Bosch F7 328 fault code mean?
The F7 fault code means your Worcester Bosch boiler has detected an ionisation signal — in other words, it thinks a flame is present — at a moment when the gas valve should be firmly closed. Because this represents a potential safety concern, the boiler immediately locks out and stops heating until the issue is investigated. The display will show F7 alongside a flashing reset indicator. Worcester Bosch uses numeric sub-codes to give engineers more detail about what triggered the lockout. The two you are most likely to see are: • F7 228 — An ionisation current was detected before the burner had even attempted to start. This points firmly at a hardware issue: a contaminated or damaged electrode assembly, a wiring fault, or a faulty PCB misreading the sensor signal. • F7 328 — A brief interruption to the mains power supply was recorded. This sub-code is distinct from the ionisation causes above and may clear with a single controlled reset. If it reoccurs, the incoming power supply or boiler wiring should be checked by a qualified engineer.
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
- Dirty or damaged ignition/ionisation electrode Common
The electrode assembly both ignites the gas and detects whether a flame is present via a tiny ionisation current. Carbon deposits, physical wear, or a hairline crack in the ceramic insulator can cause the electrode to leak a small current even when no flame exists, tricking the boiler into registering a phantom flame. This is by far the most frequent root cause of an F7 228 lockout.
- Wiring or connector fault on the flame-sensing circuit Sometimes
The low-voltage signal from the ionisation electrode travels through a dedicated lead to the PCB. Corrosion on a connector, a pinched cable, or moisture ingress can introduce stray electrical signals that mimic a genuine flame reading, triggering the F7 lockout even though the electrode itself is fine.
- Brief mains power interruption (F7 328 specifically) Sometimes
The F7 328 sub-code is logged when the boiler detects a momentary drop or loss in mains supply voltage. This can be caused by a local power cut, a loose connection at the fused spur, or voltage fluctuations on the supply. Unlike the ionisation-related causes, this sub-code often clears after a single reset and does not necessarily indicate a component fault inside the boiler.
- Flue restriction or combustion gas anomaly Sometimes
A partially blocked or incorrectly installed flue can allow combustion gases to linger in the combustion chamber. Elevated CO₂ levels in this space can interfere with the ionisation measurement, causing the boiler to misread the sensor and log an F7 fault. A Gas Safe engineer can check flue integrity and measure combustion gases.
- Faulty PCB Rare
If the printed circuit board's ionisation-measurement circuitry is damaged — for example by a voltage spike — it can generate a false flame signal internally, with no fault in the electrode or its wiring. PCB failure accounts for a small minority of F7 faults and is usually only diagnosed after all other components have been ruled out.
How to fix it
- Check the display and note the full code DIY safe
Before doing anything else, confirm exactly what is shown on the display. Note whether it reads F7 alone, F7 228, or F7 328 — the sub-code matters for diagnosis. Also check that the reset indicator is flashing, confirming a lockout state rather than just an information message.
- Attempt a single controlled reset DIY safe
Press and hold the reset button on your Worcester Bosch boiler for approximately three seconds until the display responds. Alternatively, switch the boiler off at its dedicated fused spur, wait two minutes, and switch it back on. This is particularly worth trying if your display shows F7 328, which can be caused by a brief power blip. Do not attempt more than one or two resets — if the lockout returns immediately, further resets will not help and could mask a genuine safety issue.
- Check that your gas supply is working DIY safe
Confirm that other gas appliances in your home (hob, gas fire) are working normally. If they are not, contact your gas supplier rather than an engineer, as the problem may be with the incoming supply rather than the boiler itself.
- Do not open the boiler casing or touch internal components Gas Safe engineer
Internal inspection and cleaning of the electrode assembly, flue system, wiring, PCB, and gas valve must only be carried out by a Gas Safe registered engineer. Opening the casing yourself is unsafe, may invalidate your warranty, and is not permitted under UK gas safety regulations.
- Call a Gas Safe registered engineer for full diagnosis Gas Safe engineer
If the boiler locks out again after your reset attempt, a qualified engineer needs to attend. They will inspect and clean or replace the ionisation/ignition electrode assembly, check all wiring and connectors on the flame-sensing circuit, test flue integrity and combustion gas levels, and — if those checks pass — test or replace the PCB. Annual servicing can prevent electrode-related faults like this from occurring in the first place.
Parts you may need
- Ignition/ionisation electrode assembly · from £65
- Electrode lead / ionisation cable · from £25
- PCB (main 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 £100–£300, depending on the underlying cause.
Frequently asked questions
What is the difference between F7 228 and F7 328 on a Worcester Bosch boiler?
F7 228 means the boiler detected an ionisation current before the burner started — pointing to a hardware fault such as a dirty electrode, a wiring problem, or a PCB issue. F7 328, by contrast, means the boiler logged a brief mains power interruption. The 328 sub-code will sometimes clear after a single reset and may not indicate any internal component fault. If you are unsure which sub-code you have, note it down carefully before calling an engineer — it will help them prepare for the visit.
Can I reset a Worcester Bosch F7 fault myself?
You can attempt one controlled reset — press the reset button for three seconds or cycle the power at the fused spur. This is reasonable to try once, especially if you see F7 328 (power interruption). However, if the lockout returns after your reset, do not keep trying. The boiler is protecting itself for a reason, and repeated resets without fixing the underlying fault can damage components or mask a safety issue. Call a Gas Safe registered engineer if the fault persists.
How much does it cost to fix a Worcester Bosch F7 fault?
For most homeowners, the repair involves cleaning or replacing the ignition and ionisation electrode assembly. Including parts and labour, this typically costs between £100 and £200. If the fault turns out to be wiring-related the cost is usually similar. A full PCB replacement is the most expensive scenario and can cost £300–£550 including labour — but this is uncommon, and a good engineer will only recommend it after ruling out the simpler causes first.
Will an annual boiler service prevent the F7 fault code?
Quite possibly, yes. The most common cause of an F7 lockout is a contaminated or worn electrode assembly — exactly the kind of component a qualified engineer inspects, cleans, and tests during a standard annual service. Regular servicing also keeps your warranty valid on Worcester Bosch boilers and is the single most effective way to catch deteriorating components before they cause a breakdown in cold weather.