Buderus 9U 233 Fault Code: Causes, Fixes & Repair Costs
What does the Buderus 9U 233 fault code mean?
The 9U 233 fault code on Buderus boilers (including the Logamax GB series) is made up of two elements. The '9U' portion indicates a problem with the KIM — the Kessel-Identifikations-Modul, or Boiler Identification Module — a small data card housed inside the Universal Burner Automatic (UBA) control unit. This card stores the boiler's unique configuration and identity data, which the UBA needs to operate correctly. The '233' sub-code narrows this down further to a code plug or Heat Control Module (HCM) fault, meaning the UBA cannot properly read or communicate with the KIM card or the HCM. Because Buderus is part of the Bosch Group and shares the same EMS/UBA control platform as Worcester Bosch, this code can appear on Worcester Bosch boilers too, where it is described as an HCM or code plug error. In all cases, the root issue is that the boiler's control electronics have lost reliable communication with the module that tells it exactly what type of boiler it is and how it should behave — so it locks out as a safety measure.
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
- KIM card not properly seated in the UBA Common
This is the single most frequent cause. The KIM card can appear fully inserted in its sub-base holder but may actually have been pushed too far back, preventing proper contact with the UBA. The card needs to be removed from the sub-base holder and inserted directly into the UBA before the UBA is mounted onto the base — a simple physical fix, but one that requires a qualified engineer to access the control unit safely.
- Defective KIM card Common
The KIM card itself can fail electronically over time, meaning it can no longer supply the boiler's configuration data to the UBA. In this case, the card needs to be replaced with a correctly programmed unit for that specific boiler model.
- Loose or incorrect wiring to the UBA or gas valve assembly Sometimes
Loose, corroded, or incorrectly routed wiring to the UBA or the gas valve assembly can disrupt communication between components and trigger this code. An engineer will check the wiring harness and all associated connectors during diagnosis.
- Defective or loose Heat Control Module (HCM) / code plug Sometimes
The HCM or code plug — which works alongside the KIM to configure boiler behaviour — may have developed a fault or become partially dislodged. The engineer will inspect and test it, replacing it if necessary.
- Faulty gas burner fitting cable connection Sometimes
A damaged or poorly connected cable at the gas burner fitting can also disrupt UBA communication and contribute to this fault code appearing alongside the KIM error.
- Defective UBA (Universal Burner Automatic) control unit Rare
If the KIM card, wiring, and HCM all check out correctly, the UBA control unit itself may have failed. Replacing the UBA is a more involved repair and is typically only diagnosed after other causes have been ruled out.
How to fix it
- Reset the boiler once (twice at most) DIY safe
Press the reset button on the boiler's control panel to attempt to clear the fault. If the code clears and the boiler runs normally, monitor it closely over the next few hours. If the 9U 233 code returns, do not keep pressing reset — attempting more than two or three resets in a row risks damaging the ignition transformer and will not solve an underlying hardware problem.
- Check the boiler's power supply and gas supply DIY safe
Confirm that the boiler is receiving mains power and that the gas supply to your property is on (check other gas appliances such as a hob). This rules out a simple supply interruption before calling an engineer.
- Call a Gas Safe registered engineer to inspect and reseat the KIM card Gas Safe engineer
A qualified engineer will access the UBA control unit, remove the KIM card from the sub-base holder, and re-insert it correctly into the UBA before remounting the assembly. This is the most common fix and does not always require new parts — but it must be done by a Gas Safe registered engineer as it involves working inside the boiler's electrical control system.
- Engineer to check all wiring, connectors, and the HCM/code plug Gas Safe engineer
If reseating the KIM card does not resolve the fault, the engineer will inspect the wiring harness to the gas valve and UBA, test the Heat Control Module and code plug for correct operation, and check for any loose or corroded connections throughout the control system.
- Engineer to replace the faulty component (KIM card, HCM, wiring harness, or UBA) Gas Safe engineer
Once the root cause is identified, the engineer will replace whichever component has failed. This could be the KIM card, the HCM/code plug, a damaged wiring harness, or — in rare cases — the UBA unit itself. Replacement parts must be compatible with the specific Buderus or Worcester Bosch model.
- If in doubt or repairs are costly, consider a boiler replacement assessment DIY safe
On older Buderus boilers, the cost of replacing a UBA or HCM can approach or exceed the value of the boiler itself. Ask the engineer to give you an honest assessment of the boiler's overall condition and remaining lifespan so you can weigh up repair versus replacement.
Parts you may need
- KIM card (Boiler Identification Module) · from £55
- UBA3 (Universal Burner Automatic) control unit · from £280
- Heat Control Module (HCM) / code plug · from £120
- Wiring harness / gas valve cable assembly · from £65
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 £120–£400, depending on the underlying cause.
Frequently asked questions
Can I fix the Buderus 9U 233 fault myself?
No. While you can safely reset the boiler once or twice and check that your gas supply is on, all further work on this fault involves the boiler's internal control electronics — specifically the UBA, KIM card, and HCM — which must only be handled by a Gas Safe registered engineer. Tampering with these components without the correct qualifications is both unsafe and unlawful in the UK.
How much does it cost to repair a Buderus 9U 233 fault in the UK?
If the fix is simply reseating the KIM card (no new parts needed), you would typically pay a standard call-out and labour charge of around £80–£150. If a component needs replacing — such as the KIM card or HCM — expect a total bill in the region of £200–£400 including parts and labour. A full UBA replacement is less common but can cost more; if your engineer finds that the UBA has failed on an older boiler, it is worth asking them to assess whether a full boiler replacement would be more economical.
Does the 9U 233 code appear on Worcester Bosch boilers as well?
Yes. Buderus and Worcester Bosch are both part of the Bosch Group and share the same EMS/UBA3 control platform. The 9U 233 code can appear on compatible Worcester Bosch models and carries the same meaning — a fault with the HCM code plug or the boiler identification module. The diagnosis and repair process is identical across both brands.
Is the 9U 233 fault dangerous?
The boiler locks itself out as a precaution when it cannot read its own configuration data, so it will not fire up while the fault is active. This is actually a safety feature. However, you should not repeatedly attempt to reset a locked-out boiler, and you should call a Gas Safe registered engineer promptly rather than leaving the fault unresolved — especially if you are without heating or hot water in cold weather.