Worcester Bosch F0 239 Fault Code: Causes, Fixes & Repair Costs
What does the Worcester Bosch F0 239 fault code mean?
The F0 239 fault code on a Worcester Bosch boiler signals an internal error within the Burner Control Unit, also known as the KIM (Boiler Identification Module). The boiler's control system has detected something wrong internally and has shut down normal heating and hot water operation as a precaution. The 'F0' portion of the code tells you the fault category is an internal control failure, while the '239' sub-code pinpoints the issue more specifically to the burner control unit or its associated electronics. In plain terms, the brain of the boiler has flagged a problem it cannot resolve on its own — this is not a fault that will simply go away with repeated resets.
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 failed Burner Control Unit (KIM) Common
The KIM is a dedicated module that manages ignition and burner sequencing. When it develops an internal fault — due to age, component degradation, or electrical stress — the boiler logs F0 239 and locks out. This is the most frequently identified root cause when an engineer diagnoses this specific sub-code, and replacement of the KIM or PCB assembly is often the outcome.
- Loose, damaged, or corroded wiring connections Common
Poor electrical contact between the burner control unit and the main PCB is a very common trigger. Wiring looms can develop micro-breaks, connectors can corrode over time, or plugs can work themselves slightly loose after years of vibration and thermal expansion. An engineer will check these connections before condemning the KIM itself.
- Moisture or water ingress near the electronics Sometimes
Even a minor internal leak — from the pump seal, a heat exchanger joint, or a pressure relief valve — can allow water or humidity to reach the PCB or KIM. Moisture damage to electronics often presents as an intermittent fault at first, then a permanent lockout. This is worth investigating if there is any history of dripping or damp inside the boiler casing.
- Power surge or supply disturbance Sometimes
A sudden spike or interruption in the mains supply can corrupt the control board's firmware or damage sensitive electronic components. If the F0 239 fault appeared shortly after a power cut or storm, this could be a contributing factor, though it often leads to the same outcome — KIM or PCB failure.
- Faulty sensor causing a cascade fault Rare
On rare occasions, a defective temperature or pressure sensor sends corrupted data to the control unit, causing the KIM to register an internal error rather than a straightforward sensor fault code. A thorough engineer diagnosis will rule this out or identify it early.
How to fix it
- Attempt a single boiler reset DIY safe
Press and hold the reset button on your Worcester Bosch boiler as described in your user manual. Allow the boiler to run through its start-up sequence. If it fires up normally and stays running, monitor it closely over the next 24 hours. If the F0 239 code returns — even once — do not reset again and move on to the next step.
- Note down the exact fault code and recent boiler history DIY safe
Write down the full code as shown on the display (F0 239), the time and date it appeared, whether heating or hot water stopped, and whether there was a recent power cut or unusual noise before the fault. This information will help your engineer diagnose the fault more quickly and could save you labour time.
- Check that the gas supply to the property is working DIY safe
Confirm that other gas appliances in your home (such as a gas hob) are working normally. If there is no gas supply, contact your gas supplier rather than an engineer. If gas is available elsewhere but the boiler still shows F0 239, the fault is internal to the boiler electronics, not the supply.
- Do not remove the boiler casing or inspect internal wiring Gas Safe engineer
The F0 239 fault is rooted in the control electronics. Inspecting or touching internal wiring, connectors, the PCB, or the KIM requires the correct tools, training, and Gas Safe registration. Attempting this yourself is both unsafe and illegal in the UK — leave the internal diagnosis entirely to a qualified engineer.
- Have a Gas Safe registered engineer carry out a full diagnostic inspection Gas Safe engineer
An engineer will remove the casing safely, inspect the wiring loom and connectors to the KIM, check for signs of moisture ingress or heat damage, and test the burner control unit using specialist diagnostic equipment. They will determine whether the fault lies in the wiring connections, the KIM module itself, or the main PCB, and advise on the most cost-effective repair.
- Arrange replacement of the KIM or PCB if confirmed faulty Gas Safe engineer
If the engineer confirms the burner control unit or PCB has failed, they will source the correct replacement part for your specific boiler model and carry out the swap. This is a Gas Safe notifiable task involving the boiler's control system. Once replaced, the engineer will test the boiler through a full operating cycle to confirm the fault has been resolved.
Parts you may need
- Burner Control Unit (KIM module) · from £220
- Main PCB (Printed Circuit Board) · from £280
- Wiring loom / cable harness · from £45
- Boiler connector plugs and terminals · from £15
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–£450, depending on the underlying cause.
Frequently asked questions
Can I keep resetting my boiler to get it working while I wait for an engineer?
You should attempt no more than one or two resets maximum. The F0 239 code points to an internal control fault, and repeatedly resetting the boiler does not fix the underlying problem — it simply delays diagnosis and could potentially cause further damage to the electronics. If the fault returns after a single reset, switch the boiler off at the programmer or room thermostat and wait for a Gas Safe engineer.
How much does it typically cost to fix a Worcester Bosch F0 239 fault?
Most homeowners pay somewhere between £150 and £450 all-in, depending on what the engineer finds. If the issue is loose or corroded wiring connections, the repair is usually at the lower end of that range. If the KIM module or main PCB needs replacing, the total cost including labour will typically be £300–£450. In unusual cases where a new PCB is required for a less common model and parts are hard to source, costs can exceed £500 — but this is not the typical outcome.
My Worcester Bosch boiler is over 10 years old and showing F0 239 — is it worth repairing?
This is a genuinely important question. If the engineer confirms the PCB or KIM has failed and your boiler is older than 10–12 years, it is worth considering whether repair or replacement makes more sense. A PCB replacement on an ageing boiler can cost £300–£450, whereas a new A-rated boiler installation typically starts around £1,800–£2,500 and will be far more efficient. Ask your engineer for an honest assessment of the boiler's overall condition before committing to a major repair.
What is the KIM (Burner Control Unit) on a Worcester Bosch boiler?
KIM stands for Boiler Identification Module in Worcester Bosch terminology. It is a dedicated electronic component that controls the ignition sequence and burner operation, communicating with the main PCB to manage how and when the boiler fires. When this module develops a fault, the boiler cannot safely attempt ignition and logs the F0 239 code to alert an engineer to the specific problem. Replacing it requires a Gas Safe registered engineer.