Worcester Bosch F0 238 Fault Code: Causes, Fixes & Repair Costs
What does the Worcester Bosch F0 238 fault code mean?
The F0 prefix on a Worcester Bosch boiler signals an internal component failure that has caused the boiler to lock out as a safety precaution. Sub-code 238 specifically points to a problem with the gas valve coils or the burner control unit — sometimes called the KIM (boiler identification module). In plain terms, the boiler has detected that either the gas valve is not responding correctly, or the component responsible for managing gas valve operation has developed a fault. Because this directly affects how gas is controlled inside the appliance, the boiler shuts down automatically rather than risk unsafe operation. This code can also surface alongside a PCB or internal wiring problem, so a Gas Safe engineer will need to test several components to pinpoint the exact cause.
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 gas valve coils Common
The gas valve contains electromagnetic coils that open and close to control gas flow into the burner. When one or more coils fail — due to age, electrical surge, or wear — the boiler's control system detects the incorrect response and triggers F0 238. This is the most frequently identified root cause of this specific sub-code.
- Burner control unit (KIM) failure Common
The KIM is the module that coordinates ignition sequencing and communicates with the gas valve. If this unit develops an internal fault, it can send incorrect signals or fail to communicate with the valve altogether, producing the F0 238 lockout. The boiler cannot safely fire without a functioning control unit.
- PCB or internal wiring fault Sometimes
The main printed circuit board governs all electronic communication within the boiler. A failed solder joint, damaged track, or corroded connector on the PCB can mimic a gas valve fault and trigger an F0 code. Similarly, a chafed or loose wire in the loom connecting the PCB to the gas valve will produce the same error.
- Gas valve leaking or mechanically stuck Sometimes
Physical damage or corrosion can cause the valve body itself to leak internally or become stuck in one position. This prevents correct gas flow regulation, which the boiler detects and flags as an F0 238 fault. A gas leak of any kind is a safety emergency requiring immediate action.
- Disrupted gas supply or very low system pressure Rare
While not a direct cause of an F0 238, an interrupted gas supply or system pressure that has dropped well below 1 bar can contribute to ignition failures that eventually produce internal fault codes. Always rule these out first before assuming a component has failed.
How to fix it
- Check your gas supply is active DIY safe
Turn on a gas hob burner or another gas appliance in your home. If you get a flame or confirm gas is flowing, your supply is fine. If there is no gas at any appliance, contact your gas network provider (Cadent, Wales & West, SGN, or Northern Gas Networks depending on your region). Do not attempt to work on any gas pipework yourself.
- Check the boiler system pressure DIY safe
Look at the pressure gauge on the front of your boiler. It should read between 1 and 1.5 bar when the system is cold. If it has fallen below 1 bar, you can top it up using the filling loop — consult your boiler manual for the exact procedure for your model. Once pressure is back in range, note it down and monitor whether it drops again, as a recurring pressure loss suggests a separate leak.
- Attempt a single boiler reset DIY safe
Press and hold the reset button on your Worcester Bosch boiler for roughly three seconds (the exact location varies by model — check your manual). If the boiler fires up and runs normally, monitor it over the next hour. If F0 238 returns, do not keep resetting. Repeated resets on a locking fault can mask the underlying problem and risk further damage. One reset attempt is enough at this stage.
- If you smell gas or suspect a leak, leave immediately DIY safe
Do not attempt any further checks. Evacuate the property, avoid operating light switches or anything electrical, and call the National Gas Emergency Service on 0800 111 999 (free, 24 hours). Only return once the property has been declared safe.
- Book a Gas Safe registered engineer to diagnose the fault Gas Safe engineer
Because F0 238 relates to the gas valve coils or burner control unit, all further investigation must be carried out by a Gas Safe registered engineer. They will use specialist diagnostic equipment to test the gas valve coil resistance, inspect the KIM module, and check the PCB and wiring loom. Always ask to see an engineer's Gas Safe ID card before any work begins — you can also verify registration at gassaferegister.co.uk.
- Allow the engineer to repair or replace the faulty component Gas Safe engineer
Depending on their findings, the engineer may replace the gas valve or its coil assembly, renew the burner control unit, repair a wiring fault, or replace the PCB. Gas valve work must only be carried out by a Gas Safe registered engineer — it is illegal and dangerous for an unqualified person to work on gas fittings. Ensure you receive a signed Gas Safe certificate for any gas work carried out.
Parts you may need
- Gas valve assembly (Worcester Bosch compatible) · from £120
- Gas valve coil kit · from £45
- Burner control unit / KIM module · from £95
- Main PCB (model-dependent) · from £220
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–£420, depending on the underlying cause.
Frequently asked questions
Is F0 238 dangerous — should I stop using my boiler immediately?
Yes, you should leave the boiler locked out and not attempt to keep resetting it. Because F0 238 relates to gas valve control, the boiler has shut itself down as a safety measure. Using the boiler while this fault is active is not advisable. If at any point you smell gas, leave the property and call 0800 111 999 immediately.
What is the difference between the F0 fault and the sub-code 238?
On Worcester Bosch boilers, the display typically shows two pieces of information: the fault category (F0, meaning an internal component failure) and a sub-code (238) that identifies exactly which component or circuit is involved. Sub-code 238 specifically points to the gas valve coils or the burner control unit (KIM). Other F0 sub-codes point to different components — for example, F0 222 relates to a flame detection issue and F0 224 to a gas valve drive fault — so the sub-code is essential for accurate diagnosis. Always tell your engineer both numbers.
How much does it cost to fix a Worcester Bosch F0 238 fault in the UK?
For most households, the repair falls between £150 and £420 including parts and labour. A gas valve coil replacement tends to sit at the lower end of that range, while a full gas valve or burner control unit replacement will cost more. If the PCB is at fault, that is typically the most expensive outcome — PCB replacements can reach £500–£700 on some models — but this is less common with F0 238 specifically. Call-out and diagnostic fees are usually £60–£120 on top if not included in the repair quote, so ask upfront.
My Worcester Bosch boiler is over 10 years old and showing F0 238 — is it worth repairing?
It depends on the overall condition and repair history of the boiler. If the boiler is over 12 years old or has needed multiple repairs in recent years, it is worth getting a replacement quote alongside the repair estimate. A new Worcester Bosch Greenstar 4000 or similar A-rated combi typically costs £2,000–£3,000 installed and comes with a manufacturer guarantee of up to 10 years, which often makes it better value in the long run than repeatedly repairing an ageing appliance.