Worcester Bosch F0 242 Fault Code: Causes, Fixes & Repair Costs
What does the Worcester Bosch F0 242 fault code mean?
The F0 242 fault code on a Worcester Bosch boiler signals a problem with the burner control unit — known as the KIM. When this code appears, the boiler has detected an issue with the KIM's wiring or its operating characteristics, and in some cases the main PCB may also be involved. The boiler will typically lock out, cutting off heating and hot water, and you'll usually see the fault indicator LED flashing alongside the RESET button. The F0 prefix covers a family of internal component faults; sub-codes in the range F0 242 through F0 272 all point specifically to the burner control unit as the source of the problem. Sub-code 242 is the most commonly reported, but if your display shows anything from 243 to 272 alongside F0, the diagnosis and repair path are essentially the same: the KIM wiring and operation need professional inspection, and replacement of the unit may be required.
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 burner control unit (KIM) Common
This is the primary and most specific cause of F0 242. The KIM is the dedicated electronic unit that manages burner ignition and operation. If it develops an internal fault or its connections degrade, the boiler will lock out and log this code. Replacement of the KIM is the most common resolution.
- Wiring or connector fault at the KIM Common
Loose, corroded, or damaged wiring between the KIM and the rest of the boiler's control system can cause the same code to appear even if the KIM itself is still functional. An engineer will check all plug connections and wiring continuity before condemning the unit.
- Damaged or failing PCB Sometimes
Worcester Bosch's own service documentation lists a damaged PCB as a concurrent cause alongside KIM faults. If the PCB has been affected by a power surge or has deteriorated over time, it can trigger or accompany an F0 fault. In some cases both the KIM and PCB require attention.
- Power surge or electrical glitch Sometimes
A sudden mains voltage spike or brief power interruption can cause the boiler's control electronics to log a spurious F0 fault. If the code clears after a single reset and does not return, a one-off electrical event is the likely explanation. If it persists, assume a genuine component fault.
How to fix it
- Note the exact code on the display and any symptoms DIY safe
Write down the full code as shown — for example 'F0 242' — and record whether heating, hot water, or both have stopped. Note if there was a recent power cut or electrical storm. This information will help your engineer understand the fault history and potentially speed up diagnosis.
- Attempt a single reset DIY safe
Press and hold the boiler's reset button for around three seconds, then release. If the fault clears, monitor the boiler closely. If F0 242 reappears, do not keep resetting — repeated resets will not fix an underlying component fault and may mask useful diagnostic information. Two to three attempts is the absolute maximum.
- Do not remove the boiler casing or inspect internal components Gas Safe engineer
Internal checks on the KIM, PCB, wiring looms, or any other electronic component must only be carried out by a Gas Safe registered engineer. Opening the casing yourself is unsafe, could void any remaining warranty, and is not permitted for gas appliances under UK regulations.
- Call a Gas Safe registered engineer to diagnose and repair the fault Gas Safe engineer
An engineer will test the KIM wiring and plug connections, verify the unit's operating characteristics, and replace the burner control unit if testing confirms it has failed. They will also inspect the PCB for any associated damage. This is not a fault that resolves without professional intervention.
Parts you may need
- Burner control unit (KIM) · from £120
- Main PCB · from £280
- KIM wiring harness / connector set · from £35
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–£380, depending on the underlying cause.
Frequently asked questions
Can I reset a Worcester Bosch F0 242 fault myself?
Yes, one reset is worth trying — hold the reset button for about three seconds. If the code disappears and stays away, an electrical glitch may have been the cause. However, if F0 242 comes back, stop resetting and book an engineer. Repeatedly resetting the boiler will not fix a faulty KIM or PCB, and it can make diagnosis harder.
What is the burner control unit (KIM) and how much does it cost to replace?
The KIM is a dedicated electronic module that controls the ignition sequence and burner operation. It sits separately from the main PCB. Having a Gas Safe engineer supply and fit a replacement KIM typically costs in the region of £150–£350 depending on the specific boiler model, parts availability, and your location. Labour alone is usually £80–£120 on top of the part price.
My boiler is showing F0 243 (or another number between 243 and 272) — is that the same fault?
Yes. Sub-codes F0 242 through F0 272 all relate to the burner control unit. The exact sub-code indicates a slightly different condition detected within the KIM, but the diagnosis and repair process are the same across the whole range: the engineer will check the KIM wiring and operating characteristics and replace the unit if necessary.
Could a PCB replacement be needed, and is it worth the cost on an older boiler?
A PCB replacement is occasionally required alongside or instead of the KIM, particularly if a power surge has caused wider damage. PCB replacement typically costs £450–£700 including parts and labour, which is significantly more than a KIM swap. If your boiler is over 10 years old or out of manufacturer warranty, it is worth getting a quote for a new boiler at the same time — modern A-rated condensing boilers are considerably more efficient and the long-term running cost savings can outweigh the upfront investment.