Worcester Bosch F7 Fault Code: Causes, Fixes & Repair Costs
What does the Worcester Bosch F7 fault code mean?
The F7 fault code means your Worcester Bosch boiler's ionisation circuit has detected a flame signal at a point when the burner should be off. Because the control system believes a flame is present when none should exist, the boiler immediately locks out as a safety precaution and stops all heating and hot water until the underlying cause is identified and resolved. On some models — particularly those in the Greenstar Si range — the full display reads 'F7 228', which specifically indicates a false flame signal detected before ignition begins. Both 'F7' and 'F7 228' point to the same root problem: either a component is sending a rogue signal, or gas is reaching the burner when it should not be. This is a safety-critical fault; do not attempt repeated resets or try to override the lockout.
General guidance only — not a substitute for professional advice. 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 degraded ignition/ionisation electrode Common
The combined electrode assembly handles both spark ignition and flame detection. Carbon deposits, corrosion, or physical damage to the electrode tip can cause it to send a spurious ionisation signal to the PCB even when no flame is present. Experienced engineers report this as the most frequent trigger for F7 — accounting for the vast majority of cases they attend.
- Faulty flame sensor (ionisation probe) Common
Even when the electrode itself looks clean, the sensing circuit can develop an electrical fault — a cracked ceramic insulator, a damaged lead, or a poor earth connection — that produces a false positive flame reading. The PCB then faithfully reports F7 because it genuinely believes it has detected a flame.
- PCB (printed circuit board) fault Sometimes
If the electrode assembly and its wiring check out correctly, the PCB itself may be misinterpreting the ionisation signal — either due to component failure on the board or moisture ingress into its circuitry. PCB faults are less common but must be considered once other components have been ruled out.
- Gas valve passing (letting by) Sometimes
A gas valve that does not fully close allows a small amount of gas to reach the burner chamber even when the boiler is off. The residual gas can produce a weak flame or at least enough ionisation current to trigger F7. This is sometimes described as the valve 'letting by' and is a gas-safety concern that must be addressed by a Gas Safe engineer.
- Flue blockage or combustion issue Rare
A partially blocked or incorrectly installed flue can cause unusual combustion behaviour, including lingering combustion gases near the electrode that mimic a flame signal. Elevated CO₂ levels measured in the flue are a useful diagnostic indicator for engineers investigating this cause.
- Moisture or damp affecting ignition components Rare
On some Greenstar models, condensation on the electrode or its connections after a cold start or a recent reset can briefly create a conductive path that mimics an ionisation signal. This tends to clear quickly, but if F7 reappears after drying, a deeper electrical fault is likely present.
How to fix it
- Attempt a single boiler reset DIY safe
Press and hold the reset button on your Worcester Bosch boiler for approximately three seconds until the display responds, then release. Alternatively, switch the boiler off at the mains, wait two to three minutes, and switch it back on. Only attempt this once. If F7 returns immediately or reappears within the same day, do not reset again — call a Gas Safe engineer instead. Repeated resets on a false-flame fault will not fix the underlying problem and may mask a genuine gas-safety issue.
- Check for a gas smell before doing anything else DIY safe
Before and after the reset, check whether you can detect the smell of gas near the boiler or anywhere in the property. If you can smell gas at any point, do not reset or touch the boiler. Leave the building, avoid using any electrical switches, and call the National Gas Emergency Service immediately on 0800 111 999 (free, 24 hours a day).
- Note any additional fault codes on the display DIY safe
If your display is showing a secondary number — such as F7 228 — or cycling through multiple codes, make a note of everything you see and pass this information to the engineer. The sub-code F7 228 specifically points to a false flame signal before ignition, which helps the engineer prioritise the electrode assembly and gas valve in their diagnosis.
- Inspect and clean or replace the electrode assembly Gas Safe engineer
A Gas Safe engineer will remove the burner access panel, extract the electrode assembly, and examine the tip for carbon fouling, cracking, or physical damage. In many cases a thorough clean restores correct operation; if the ceramic insulator is cracked or the tip is badly corroded, a replacement electrode is fitted. This is the first component an engineer should investigate given how frequently it is responsible for F7.
- Test the ionisation circuit and wiring Gas Safe engineer
The engineer will use a multimeter to check the resistance of the ionisation lead and verify the electrode earth path. A broken or shorted lead, or a poor earth, can generate a continuous false signal regardless of whether the electrode itself is in good condition. Any damaged wiring or connectors should be replaced at this stage.
- Check the flue for blockages and test combustion Gas Safe engineer
The engineer should inspect the flue terminal for obstructions (bird nests, debris, ice in cold weather) and verify that the flue route is correctly installed with no dips that could trap gases. Measuring CO₂ levels at the flue outlet can confirm whether combustion products are loitering near the electrode.
- Test the gas valve for internal leakage Gas Safe engineer
Using appropriate test equipment, the engineer will verify that the gas valve seats correctly and is not passing gas to the burner when it should be fully closed. A valve that lets by must be replaced — it is a direct gas-safety hazard and a plausible cause of persistent F7 faults.
- Assess the PCB if other components are serviceable Gas Safe engineer
If the electrode, wiring, flue, and gas valve have all been checked and found to be in order, the PCB is the remaining suspect. The engineer may test the ionisation signal path on the board or, in many cases, replace the board as the most practical diagnostic step. PCB replacement is one of the more expensive repairs on this fault.
- Call a Gas Safe registered engineer if the fault persists after your single reset Gas Safe engineer
If F7 has not cleared after one reset, or it returns at any point, you need a Gas Safe registered engineer to carry out the checks above. You can verify any engineer's registration at gassaferegister.co.uk. Do not attempt to open the boiler casing, touch the gas valve, or interfere with any internal components yourself.
Parts you may need
- Ignition/ionisation electrode assembly · from £35
- Electrode lead and connector · from £20
- Gas valve · from £180
- PCB (main control board) · from £250
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
Can I keep resetting the boiler to clear the F7 code?
You can try a single reset, but that is the limit of what is safe to do yourself. F7 is a false-flame detection fault — meaning the boiler's safety system believes it has spotted a flame when there should not be one. Repeatedly resetting it does not address whatever is causing that rogue signal, and in a worst-case scenario (such as a gas valve letting by) it could be masking a genuine safety concern. If F7 comes back after one reset, call a Gas Safe engineer.
What does 'F7 228' mean on a Worcester Bosch boiler?
The number 228 is a sub-code that narrows the diagnosis down to a false flame signal detected before the ignition sequence has even started. This is the most specific form of F7 and strongly implicates either the ionisation electrode or its wiring sending a spurious signal, or the gas valve failing to close fully so a residual flame or gas is present in the combustion chamber before the boiler attempts to light. An engineer attending an F7 228 fault will typically start with the electrode assembly and then work through to the gas valve if necessary.
How much does it typically cost to fix a Worcester Bosch F7 fault?
Most homeowners pay somewhere between £120 and £380 to resolve an F7 fault, with the majority of cases falling in the lower half of that range because the cause is a dirty or faulty electrode — a relatively inexpensive part that can often be cleaned rather than replaced. Labour from a Gas Safe engineer typically accounts for the bulk of the bill. If the diagnosis points to a gas valve or PCB, you should expect to pay toward or beyond the top of that range; in those cases it is worth getting a boiler replacement quote alongside the repair quote, especially if the boiler is over ten years old.
Is the F7 fault dangerous?
F7 is a safety lockout, which means the boiler has already done the right thing by shutting itself down. In that sense the immediate risk has been contained. However, the underlying cause — particularly a gas valve that is not fully closing — can be a genuine gas-safety concern, so the fault should be treated with urgency. If at any point you smell gas, do not touch the boiler. Leave the property and call the National Gas Emergency Service on 0800 111 999 immediately.