Worcester Bosch F7 228 Fault Code: Causes, Fixes & Repair Costs

Common causes

• Dirty or degraded ignition/ionisation electrode assembly Common

  The ionisation electrode shares its job with the ignition electrode assembly on many Worcester Bosch models. Over time, carbon deposits, oxidation, or physical wear on the electrode tip can cause the electrode to conduct a small residual current even when no flame is present, tricking the control board into registering a false flame signal. This is the most frequently found cause during engineer diagnostics on F7 228 call-outs.

• Faulty or short-circuiting flame sensor (ionisation sensor) Common

  The flame sensor itself may have developed an internal fault — a partial short to earth or damaged insulation on the sensor lead — that produces a spurious low-level current. The PCB cannot distinguish this from a genuine ionisation signal, so it logs F7 228 and locks out as a safety precaution.

• Moisture or condensation on the electrode assembly Sometimes

  On some Greenstar models, condensation forming inside the combustion chamber — particularly after a prolonged period of inactivity or a very cold spell — can bridge across the electrode insulator and create a weak ionisation-like current before the burner fires. This can sometimes clear after a single reset but will recur if the root cause (such as a compromised flue terminal allowing downdraught or a poorly sealed combustion chamber) is not fixed.

• Flue blockage or restriction affecting combustion conditions Sometimes

  A partial blockage of the flue or air-intake — caused by debris, bird nesting material, or frost at the terminal — can affect the air-to-gas ratio inside the burner. In some cases this leads to residual combustion products in the chamber being detected by the ionisation sensor as a low-level signal before the next firing cycle begins.

• PCB (printed circuit board) fault Rare

  If the PCB's signal-processing circuitry develops a fault — damaged components, failed solder joints, or moisture ingress onto the board — it may misinterpret the very low background electrical noise from the sensor wiring as a genuine flame signal. This is usually only reached as a diagnosis after the electrode, sensor, and wiring have been checked and cleared.

How to fix it

1. Reset the boiler once DIY safe

   Press and hold the reset button (usually marked with a flame/reset symbol) for around 3 seconds, then release. Allow the boiler a full minute to attempt a restart. If it fires and runs normally, monitor it over the next few hours. If the F7 228 code returns, do not reset again — repeated resets on a lockout fault can mask a genuine safety issue and will not fix an underlying component problem.

2. Check that your gas supply is active DIY safe

   Confirm other gas appliances in the property (hob, fire) are working normally. If there is no gas elsewhere, contact your gas supplier. This is not the cause of F7 228 specifically, but ruling it out takes seconds and avoids an unnecessary engineer visit.

3. Visually inspect the flue terminal for obvious blockages DIY safe

   From outside the property, look at the flue outlet (typically a white plastic or stainless terminal on an external wall or roof). Check for visible obstructions such as a bird's nest, ice build-up, or debris sitting over the terminal. Do not attempt to dismantle or reposition the flue — if a blockage is inside the flue duct, that requires a Gas Safe engineer.

4. Call a Gas Safe registered engineer to inspect the electrode assembly Gas Safe engineer

   A qualified engineer will remove and inspect the ionisation and ignition electrode assembly. Electrodes that are worn, cracked, or heavily carbonised will be cleaned with fine wire wool or replaced outright. Worcester Bosch recommends replacing the electrode set approximately every five years as part of a service schedule. A replacement electrode set typically costs in the region of £55–£60 for parts, plus labour.

5. Engineer to check the flame sensor lead and wiring continuity Gas Safe engineer

   The engineer will test the resistance and insulation integrity of the sensor lead from the electrode back to the PCB. Any cracked insulation or chafed wiring that is shorting to the boiler casing can generate exactly the spurious current that triggers F7 228, and the wiring loom or connector may need replacement.

6. Engineer to inspect the flue system and measure flue gases Gas Safe engineer

   The engineer will check the flue for restrictions or leaks and measure CO₂ levels at the flue outlet. Abnormal combustion readings can indicate that residual gases are lingering in the combustion chamber between firing cycles, contributing to false ionisation signals. Any flue faults found must be rectified before the boiler is returned to service.

7. Engineer to test or replace the PCB if all other components pass inspection Gas Safe engineer

   If the electrode assembly, sensor lead, and flue all check out, the PCB is the remaining candidate. The engineer will test the PCB's ionisation circuit. PCB replacement is a last-resort step and carries a higher parts cost (typically £300–£550 including labour), so a competent engineer will exhaust all other possibilities first.

Parts you may need

• Ignition and ionisation electrode assembly · from £57
• Flame sensor lead / ionisation lead · from £35
• PCB (printed circuit board) · from £220
• Flue terminal / flue end cap · from £28

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