Worcester Bosch F0 279 Fault Code: Causes, Fixes & Repair Costs
What does the Worcester Bosch F0 279 fault code mean?
The F0 279 code tells you that your Worcester Bosch boiler has detected a fault specifically with the burner control unit, sometimes referred to as the KIM. This sits within the wider F0 fault family — a category of internal control errors — and the sub-code 279 narrows things down to the KIM itself or its associated wiring and electrical connections. When this code appears, the boiler's fault indicator LED will typically flash alongside (or instead of) the display reading, and the appliance will shut down normal heating and hot water operation as a safety precaution. Because the KIM governs how the burner fires and behaves, this is not a fault the boiler can work around; it will remain locked out until the underlying problem is diagnosed and resolved 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
- Failed or faulty burner control unit (KIM) Common
The KIM is the component that directly manages burner ignition and operation. When it fails internally — due to component ageing, wear, or manufacturing defect — the boiler's control system immediately flags it with the 279 sub-code. This is the most direct and common cause of this specific fault.
- Wiring fault or loose connection to the KIM Common
Damaged, corroded, or loose wiring between the KIM and the main PCB can cause the control system to read the KIM as faulty even when the unit itself is still serviceable. Vibration over time, rodent damage, or poor installation can all lead to connection issues that trigger this code.
- PCB damage or failure Sometimes
The main printed circuit board coordinates communication with the KIM. If the PCB has developed a fault — through electrical stress, moisture ingress, or component failure — it may incorrectly report a KIM fault or genuinely fail to communicate with the KIM, resulting in the F0 279 display.
- Power surge or electrical disturbance Sometimes
A sudden spike or dip in mains voltage — from a power cut, nearby lightning strike, or unstable supply — can damage sensitive electronic components including the KIM and PCB. If the fault appeared immediately after a power event, this is worth mentioning to your engineer.
- Moisture or environmental damage to electronics Rare
Condensation, a nearby leak, or prolonged exposure to heat inside the boiler casing can degrade electronic components over time. This is more common in older boilers or those installed in poorly ventilated cupboards where temperature and humidity fluctuate significantly.
How to fix it
- Note down the exact fault code displayed and when it appeared DIY safe
Write down 'F0 279' exactly as shown on your boiler display, and note the time it appeared and whether heating, hot water, or both stopped working. Also note if there was a recent power cut or electrical storm. This information helps the engineer understand the fault history before they arrive.
- Attempt a single boiler reset DIY safe
Press and hold the reset button on your boiler as described in your user manual (typically for around 3 seconds, though this varies by model). Wait for the boiler to restart and run through its startup sequence. If the F0 279 code clears and the boiler runs normally, monitor it closely over the next hour. If the code returns, do not reset again — repeated resets will not fix an internal electronic fault and may complicate the engineer's diagnosis.
- Do not remove the boiler casing or inspect any internal components Gas Safe engineer
The KIM, PCB, and associated wiring are all internal electrical components. Accessing them requires removing the boiler casing, and any work on these parts must only be carried out by a Gas Safe registered engineer. Do not attempt to unplug, inspect, or reseat any connectors inside the boiler.
- Engineer to inspect KIM wiring and electrical connections Gas Safe engineer
A Gas Safe engineer will remove the casing safely and check the wiring loom running to and from the KIM for signs of damage, corrosion, or loose terminals. Sometimes a secure reconnection is all that is needed; other times the wiring itself requires repair or replacement.
- Engineer to test and replace the burner control unit (KIM) if confirmed faulty Gas Safe engineer
Using manufacturer diagnostic tools and reference data, the engineer will assess whether the KIM is genuinely defective. If so, it will be replaced with a compatible unit. The engineer will then run the boiler through a full operational test to confirm the fault is cleared and the burner is behaving correctly.
- Engineer to assess the PCB if the fault persists after KIM replacement Gas Safe engineer
If replacing the KIM does not clear the F0 279 code, the engineer will turn their attention to the main PCB. A faulty PCB may need repair or full replacement. This is a more involved and costly job, so engineers will typically exhaust other possibilities first.
- Call a Gas Safe registered engineer to diagnose and repair the fault Gas Safe engineer
Because this fault involves internal electrical and burner-control components, a Gas Safe registered engineer must carry out all diagnostic work and repairs. You can verify an engineer's Gas Safe registration at gassaferegister.co.uk. If at any point you smell gas, leave the property immediately and call the National Gas Emergency Service on 0800 111 999.
Parts you may need
- Worcester Bosch Burner Control Unit (KIM) · from £180
- Worcester Bosch Main PCB · from £220
- Wiring loom / harness (model-specific) · from £45
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 £220–£450, depending on the underlying cause.
Frequently asked questions
What does the F0 fault family mean on a Worcester Bosch boiler?
F0 is Worcester Bosch's general category for internal control faults — problems the boiler's own electronics have detected within themselves. The sub-code that follows (in this case 279) tells an engineer which specific component or circuit is involved. Sub-code 279 points to the burner control unit (KIM). Other F0 sub-codes you may encounter include F0 261 (PCB internal fault) and F0 281 (communication error between control boards), which have their own diagnostic paths but also typically require a Gas Safe engineer to resolve.
Can I keep resetting the boiler to get heating back while I wait for an engineer?
We'd advise against it. A single reset is reasonable to check whether the fault self-clears, but if F0 279 returns after that first reset, further resets won't fix the underlying electronic fault. Repeated resets on a control-unit fault can actually make it harder for the engineer to read the fault history and pinpoint the cause quickly. Leave the boiler in lockout and book an engineer as soon as possible.
How much does it cost to fix a Worcester Bosch F0 279 fault?
Most homeowners pay somewhere between £220 and £450 for the repair, depending on whether the KIM alone needs replacing or the PCB is also involved, and including callout fee, parts, and labour. If the main PCB requires replacement on an older or less common model, costs can occasionally push higher — up to £550 or more — though this is less typical. If your boiler is over 10–12 years old and facing a repair in this range, it's worth getting a replacement boiler quote alongside the repair quote so you can compare long-term value.
Is F0 279 a Gas Safe job or can a competent DIYer fix it?
This is strictly a Gas Safe registered engineer job. While F0 279 is an electrical rather than a direct gas-leak fault, the components involved — the KIM and PCB — sit inside the boiler casing alongside gas-carrying parts. Opening the casing and working on these components without the correct qualifications is both unsafe and illegal. Always use a Gas Safe registered engineer; you can check any engineer's credentials at gassaferegister.co.uk.