9U 233 Boiler Fault Code: What It Means on Every Brand

The 9U 233 fault code appears on 2 boiler brands — Buderus and Worcester Bosch — and on all of them it points to the same kind of problem. The details still differ by brand (thresholds, sub-codes, reset procedure), so use the table below to jump to the page for your boiler.

Lockout — engineer likely 2 brands use this code

Which brand is your boiler?

The brand is printed on the front casing. Tap your brand for the full causes, fixes and repair costs.

Brand	What 9U 233 means	Severity	DIY?
BBuderus	The 9U 233 fault code on Buderus boilers (including the Logamax GB series) is made up of two elements. Full guide →	Lockout — engineer likely	No
WBWorcester Bosch	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. Full guide →	Lockout — engineer likely	No

9U 233 brand by brand

Buderus 9U 233

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.

Causes, fixes & costs for Buderus 9U 233 → · All Buderus codes

Worcester Bosch 9U 233

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.

Causes, fixes & costs for Worcester Bosch 9U 233 → · All Worcester Bosch codes

Frequently asked questions

Is the 9U 233 fault code the same on every boiler?

Broadly, yes — on Buderus and Worcester Bosch the 9U 233 code points to the same class of fault, and the recommended response is similar. Model-specific details such as sub-codes and reset procedures still vary, so follow the page for your own brand.

Can I fix a 9U 233 fault myself?

Not usually. On every brand listed, 9U 233 points to a fault that needs a suitably registered engineer to diagnose and repair. A single reset to rule out a transient glitch is reasonable; repeated resets are not.

Who should I call about a 9U 233 fault?

For gas boilers, a Gas Safe registered engineer — it is a legal requirement for any work on the appliance. If you smell gas at any point, leave the property and call the National Gas Emergency line on 0800 111 999.

General guidance only — not a substitute for professional advice, and not certified by a registered engineer. Any work on a gas appliance must be carried out by a Gas Safe registered engineer. If you smell gas, call 0800 111 999.