Worcester Bosch 9U 233 Fault Code: Causes, Fixes & Repair Costs
What does the Worcester Bosch 9U 233 fault code mean?
The 9U 233 fault code points to a problem with the code plug inside the Heat Control Module (HCM) — the electronic control unit that governs how your boiler operates. The code plug stores key configuration data specific to the boiler model; when the boiler's control system cannot read this data correctly, it locks out and displays 9U 233. The fault may also indicate that the HCM itself is defective, that an incorrect or mismatched HCM has been installed, or that a wiring fault is preventing reliable communication between the code plug and the rest of the control system. This code appears most frequently on Worcester Bosch CDi Compact boilers. Because it involves the boiler's core electronics, it is not a DIY repair — a Gas Safe registered engineer is required.
General guidance only — not a substitute for professional advice. 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 defective code plug Common
The code plug is a small component inside the HCM that stores model-specific configuration data. If this plug fails, develops a fault, or loses its data, the boiler cannot operate correctly and triggers the 9U 233 lockout. This is the most frequently identified root cause.
- Defective or loose Heat Control Module (HCM) Common
The HCM itself may have developed an internal fault — due to age, power surge, or component failure — causing it to misread or fail to communicate with the code plug. A loose or poorly seated HCM can produce the same symptom.
- Incorrect HCM fitted Sometimes
If the boiler has previously had an HCM replacement and the wrong unit was installed for the specific boiler variant, the control system will detect the mismatch and raise this fault. A related code, 9A 362, explicitly flags an HCM model mismatch, but the same underlying issue can trigger 9U 233.
- Wiring or connection fault Sometimes
Damaged, corroded, or loose wiring between the code plug and the HCM — or elsewhere in the control circuit — can interrupt data communication and cause the fault to appear even when the HCM and code plug are physically intact.
How to fix it
- Check for a simple temporary glitch by resetting the boiler DIY safe
Press and hold the reset button (typically marked with a flame and cross symbol) for around 3 seconds. Allow the boiler to restart and observe whether it fires up normally. A one-off reset is reasonable, but do not repeat this more than 2–3 times — persistent lockout means the underlying fault needs professional diagnosis.
- Check your gas supply is active DIY safe
Confirm other gas appliances in your home (hob, fire) are working normally. If there is no gas supply, contact your gas supplier. This is unlikely to be the root cause of a 9U 233 but rules out a simple external issue before calling an engineer.
- Do not attempt to access or remove the Heat Control Module yourself Gas Safe engineer
The HCM is an electronic control component connected to both the boiler's electrical and gas control systems. Accessing, adjusting, or replacing it without Gas Safe registration is unsafe and illegal under UK gas safety regulations.
- Call a Gas Safe registered engineer to diagnose the HCM and code plug Gas Safe engineer
An engineer will inspect the code plug, test the HCM, check all associated wiring and connectors, and determine whether the fault can be resolved by reseating or reprogramming the code plug, or whether the HCM needs replacing. They will also verify that the correct HCM variant is fitted for your specific boiler model.
- Have the engineer replace or reprogram the HCM or code plug as required Gas Safe engineer
If diagnosis confirms a failed code plug or defective HCM, the engineer will source the correct replacement part for your boiler variant and carry out the swap. If an incorrectly matched HCM was previously installed, the correct unit will need to be fitted and configured.
Parts you may need
- Heat Control Module (HCM) · from £280
- Code plug (HCM configuration plug) · from £45
- HCM wiring harness / loom · from £55
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 £300–£550, depending on the underlying cause.
Frequently asked questions
Can I fix the Worcester Bosch 9U 233 fault myself?
No. The 9U 233 fault involves the boiler's Heat Control Module and code plug — core electronic control components. Working on these requires specialist knowledge and Gas Safe registration. Attempting the repair yourself is unsafe and illegal in the UK. The only owner-safe action is a boiler reset (no more than 2–3 times); if the fault persists, you need a Gas Safe engineer.
How much does it cost to fix a 9U 233 fault on a Worcester Bosch boiler?
Most homeowners pay between £300 and £550 for the call-out, diagnosis, parts, and labour involved in resolving this fault. The HCM itself typically costs £200–£300 for the part, with labour on top. Worcester Bosch also offer a fixed-price out-of-warranty repair service (around £400 all-in) which can be good value if your boiler is otherwise in reasonable condition. In rare cases where additional components are also damaged, costs can climb higher — your engineer will advise after diagnosis.
What is the difference between fault codes 9U 233 and 9A 362?
Both codes relate to the Heat Control Module. The 9U 233 code indicates a general code plug error within the HCM — the module cannot correctly read the configuration data it needs to run the boiler. The 9A 362 code is more specific: it flags that the HCM fitted to the boiler does not match the correct model variant — in other words, a wrong HCM has been installed. Both require a Gas Safe engineer and often involve the same components, but 9A 362 more directly points to an incorrect part being fitted.
Is it worth repairing a Worcester Bosch boiler showing 9U 233, or should I replace it?
If your boiler is under 10 years old and in otherwise good condition, repairing the HCM is usually worthwhile — especially given Worcester Bosch's fixed-price repair option. If the boiler is over 10–12 years old, it is worth getting a replacement quote at the same time, since the cost of an HCM repair on an ageing boiler can approach a significant fraction of a new boiler installation. Your Gas Safe engineer should be able to give you an honest assessment of the boiler's overall condition to help you decide.