Worcester Bosch 9U 233 fault codewhat it means and how to fix it
The heat control module is loose, missing or faulty. This small plug-in part carries the identity of your boiler, and nothing runs without it.
Take care
Get a Gas Safe registered engineer to look at your Worcester Bosch 9U 233
We do not pass enquiries to a repair firm, so there is no form here. Find a Gas Safe registered engineer through the Gas Safe Register, and ask to see their registration card when they arrive — it is the register, not the van, that tells you they are qualified.
When did this code first appear?
The right answer depends on what you can see. Pick the one that matches.
Choose the option on the left that matches what your appliance is doing, and the answer for that case appears here.
Straight after a control board or PCB was replaced
Almost certainly the module was not transferred, not seated, or the wrong version came with the new board. This is a return visit, not a new job.
Contact the engineer who fitted it. You should not be paying twice for the same repair.
Shortly after a routine service
Panels come off and go back on during a service, and a module can be nudged out of full contact in the process.
Mention the service when you ring. Reseating it may be all that is required.
No work has been done on the boiler at all
Then contacts or the module itself are the likely candidates, which is a slightly bigger job but still a modest one.
Book a Gas Safe registered engineer to test the module and its seating.
What 9U 233 means
9U 233 concerns the heat control module, which Worcester shortens to HCM and which older documentation sometimes calls the code plug or boiler identification module. It is a small component that plugs into the control board and tells it exactly which appliance it is fitted to — the output, the range, the operating characteristics. A control board is essentially generic until the module tells it what it is driving.
Take the module out, seat it badly, or fit one from a different model, and the board has no idea what it is controlling, so it refuses to fire. That is why this code turns up disproportionately after a repair. It is far more often a consequence of work done than a component that failed on its own, which is worth knowing before anyone starts pricing up parts.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| Module not fully seated after a repair | The most frequent cause by a wide margin. It goes back in during reassembly but not quite home. | Common |
| Wrong module fitted for the appliance | A board supplied with a module for a different output leaves the boiler unable to recognise itself. | Common |
| Dirty or corroded contacts | Years in a warm damp cupboard leave a film on the pins that is enough to break communication. | Occasional |
| Module or board failure | Genuine failure happens but sits behind the simpler explanations, so it is diagnosed last. | Occasional |
Can you fix it yourself?
Stop — this needs an engineer
What it costs to fix
Typical repair cost
£80–£450
Reseating during a call-out £80–£150. Replacement module £120–£240 fitted. Control board, only where testing points to it, £350–£550.
Typical UK prices including parts and labour, updated September 2026. Your quote may differ.
Own this appliance and had this code? . Nothing is published until a person has read it.
Related Worcester Bosch fault codes
- Blue light flashingGet it checkedOn a Greenstar with no display, the blue light is the fault code. Steady means healthy; a flashing pattern, read with the reset button, is a diagnosis Worcester publish in the installation instructions.Related: electronics and power
- F0Get it checkedThe boiler has found a fault inside its own electronics and stopped. F0 covers the control board and the burner control unit, and it needs an engineer.Related: electronics and power
- A7Get it checkedThe hot water temperature sensor has failed. Heating usually carries on as normal while the hot water runs cold, scalding, or swings between the two.Related: electronics and power
- Red light flashingGet it checkedOn a Worcester built before the Greenstar era, the flashing lights are the fault code. Slow means a lockout you can reset once; fast means a fault the boiler is waiting out or an engineer's.Related: electronics and power
- Fd 231Get it checkedPower to the boiler was cut while it sat in a lockout, so a fault happened before the interruption. On older Greenstars, Fd simply means the reset button was pressed by mistake.Related: electronics and power
- 2980Stop — call an engineerSuper lock-out. Five locking faults were reset inside fifteen minutes while the boiler saw a potentially safety-critical condition, so it has locked itself against further resets. Only an engineer can clear it.Related: electronics and power
Own a Worcester Bosch boiler? Save it and its codes come first next time.
Related guides
Common questions about Worcester Bosch 9U 233
What actually is the heat control module?
A small plug-in module holding the settings and identity for your specific appliance. Think of it as the part that turns a general purpose control board into the board for your boiler. Worcester also refers to it as the KIM or code plug depending on the model and the age of the documentation.
Can I push it back in myself?
No. It sits inside the sealed casing of a gas appliance, and opening that casing is work for a Gas Safe registered engineer. It is a five minute job for someone who is allowed to do it.
Will I need a whole new board?
Often not. If the module is simply loose the fix costs a call-out. If the module itself has failed it is a modest part on its own. A full board only comes into it when testing shows the board cannot read a known good module.
Why did it appear after a repair I already paid for?
Because fitting a board means transferring or configuring this module, and it is an easy step to get wrong. Raise it with the original engineer — reputable firms treat it as unfinished work rather than a new chargeable visit.
Technical review
Not reviewed by an engineer
No engineer has read this page. Severity follows what the manufacturer publishes, and we would rather tell you that than let a byline imply a sign-off.
Sources and how we verified this
We establish the facts from the sources below and then write the explanation ourselves. We do not copy manufacturer wording and we do not host their manuals.
- Manufacturer technical and service documentation
- Manufacturer support portal
This page has not been reviewed by an engineer, and we would rather tell you that than imply otherwise. The severity rating follows what the manufacturer publishes: where they say isolate the appliance and call someone, so do we.