Worcester Bosch E2 233 fault codewhat it means and how to fix it
The heat control module, the code plug that tells the board which Heatslave II it is running, is missing, loose or faulty. 9A 360 is its sibling: a plug fitted for the wrong boiler.
Take care
Get an OFTEC registered technician to look at your Worcester Bosch E2 233
We do not pass enquiries to a repair firm, so there is no form here. Find an OFTEC registered technician through the OFTEC, and ask to see their registration card when they arrive — it is the register, not the van, that tells you they are qualified.
What happened to the boiler just before the code?
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.
It was serviced, or a part was changed
The module was disturbed or not transferred. This is the common case, and it is a five-minute correction for whoever did the work.
Call the technician who did the work back. Tell them the code and whether it is E2 233 or 9A 360, because the second means the wrong module rather than a loose one.
Nothing — it appeared on its own
A module that has failed, or a connection that has worked loose over time. Either way the board can no longer read it.
Reset once. If it returns, book an OFTEC registered technician and quote the code; a replacement module is a small part.
The display shows 9A 360
The module is present but is for a different Heatslave variant. The boiler is blocked, not locked, and stays blocked until the right one is fitted.
Nothing to reset. The technician reads the fitted module's number from the info menu, checks it against Worcester's list and fits the correct one.
What E2 233 means
Worcester's control boards are not made for one boiler. A small plug-in module — the heat control module, HCM, which older documentation calls the code plug — tells the board which appliance it is fitted to, and the Heatslave II's table reserves two codes for it. E2 233 is "HCM code plug missing or defective", a locking fault cleared with the reset once the module is seated or replaced. 9A 360 is "incorrect HCM for boiler variant", a blocking fault that holds the boiler until the right module is fitted.
For both, the manual sends the engineer to the info menu: items i22 and i23 show the number and version of the module that is fitted, and table 11 lists which number belongs to which variant — the last three digits 391, for example, mean the 12/18. The module rarely fails on its own. It comes adrift when the control box drawer has been pulled forward and tilted for a service and a plug is not pushed fully home, and it goes missing or wrong when a control board is replaced and the old module is not moved across or the wrong one is fitted from stock. That is why the question worth asking is what was done to the boiler just before the code appeared.
The gas Greenstar i and Compact ranges report the same module fault as 9U 233 and the wrong variant as 9A 362; the Heatslave II uses E2 for the first and keeps 9A for the second. The module sits inside the control box, which carries mains voltage, so reseating it is inside the case rather than a knob to turn from the front.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| Module unseated after a service or board change | The control box drawer is pulled forward and tilted for servicing, and a module not pushed fully home afterwards loses contact. After a board replacement the old module has to move to the new board, and a forgotten one reads as missing. | Common |
| Wrong module for the variant (9A 360) | A module from a different output — a 12/18 plug in an 18/25 — is present and readable but wrong, and the board blocks until the correct HCM number from Worcester's list is fitted. Info menu item i22 shows which one is in. | Occasional |
| Module failed | The module is a small electronic component and can fail outright, which the board reports exactly as a missing one. Worcester's remedy is the same: identify the variant from table 11 and fit the matching module. | Occasional |
Can you fix it yourself?
Stop — this needs an engineer
What it costs to fix
Typical repair cost
£80–£300
Time on site
45 minutes
Reseating during a call-out £80–£150. A replacement module £60–£150 fitted, chosen by variant from Worcester's list. If the code followed someone else's work, ask them to come back before paying anyone.
Typical UK prices including parts and labour, updated September 2026. Your quote may differ.
Parts you may need
We may earn a commission if you buy a part through one of these links. It does not change what you pay, and it does not affect which parts we list.
Worcester Bosch Greenstar control board (PCB)
£250–£450Authorised Worcester parts distributor
Price checked 28 September 2026
Worcester Bosch Greenstar heat control module (HCM) or code plug
£30–£90Authorised Worcester parts distributor
Price checked 28 September 2026
Own this appliance and had this code? . Nothing is published until a person has read it.
Related Worcester Bosch fault codes
- A0 809Get it checkedThe sensor reading the Heatslave II's internal hot water store has failed or lost its connection, so the boiler cannot tell how hot the store is and locks out until it is checked or replaced.Related: oil sensors and code plug
- H27Usually simple to fixA warning triangle with H27 means the sensor on the incoming cold mains has failed. Worcester say operation is not affected, so heating and hot water carry on until the sensor is replaced.Related: oil sensors and code plug
- EA 227Stop — call an engineerThe boiler tried to light and never detected a flame, so it has shut the gas off and locked out. This is the most common Worcester fault code.Related: same brand
- CE 207Get it checkedSystem water pressure has dropped too low for the boiler to run safely. Topping the system back up to 1.2 bar usually clears it in five minutes.Related: same brand
- E9 219Stop — call an engineerThe boiler has overheated and the safety thermostat has tripped. Water is not carrying heat away from the exchanger, and it should not be run until that is fixed.Related: same brand
- C6 224Get it checkedThe fan is not reaching the speed the boiler asked for, so combustion cannot be confirmed as safe and the burner has been prevented from firing.Related: same brand
Own a Worcester Bosch boiler? Save it and its codes come first next time.
Related guides
Common questions about Worcester Bosch E2 233
What is 9A 360?
The other half of this fault: incorrect HCM for boiler variant. Where E2 233 means the board cannot read a module at all, 9A 360 means it can read one and it is the wrong one. It is a blocking fault rather than a locking one, so there is nothing to reset; the boiler waits for the correct module, which Worcester identify from table 11 and info menu items i22 and i23.
Is this the same as 9U 233 on a gas Greenstar?
The same component and the same fault, under different letters. Worcester’s gas Greenstar i and Compact ranges show the module fault as 9U 233 and the wrong-variant fault as 9A 362; the Heatslave II shows E2 233 and 9A 360. Those gas pages are written for gas boilers, and this one names the technician an oil boiler needs.
Can I reseat the module myself?
It is inside the control box, behind the front panel, with 230 volts on the terminals around it, and Worcester's instruction is written for the engineer. Oil work is not restricted by law the way gas work is, but this is not a job to do from the front of the boiler. If the code followed a service, the call-back is free of charge in most cases; if it did not, it is a short visit for an OFTEC registered technician.
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
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.