U9 or 9U: what the code really is
If your Worcester boiler shows U9, the code in the manuals is 9U, paired with cause code 233, and it points to the boiler's Heat Control Module (HCM). The two characters are easy to read the wrong way round, and most people search for U9, but you will only find 9U in Worcester's fault tables. Write down both parts, 9U and 233, before you do anything else.
Worcester's Greenstar i boilers show faults as a short display code and a three-digit cause code. The display code gives the general area and the cause code pins it down, so 9U 233 is one specific fault, not a family of them.
What the Worcester manuals say about 9U 233
Two Worcester documents for the Greenstar i range describe the code, in slightly different words.
| Document | Models | How 9U 233 is described |
|---|---|---|
| Installation, commissioning and servicing instructions, Greenstar i System Compact (6 720 807 726, 2013/05) | Greenstar 27i and 30i System Compact, natural gas and LPG | "HCM (Heat Control Module)", with the possible cause given as a problem with the code plug, faulty or not connected |
| Installation, commissioning and servicing instructions, Greenstar i System ErP (6 720 806 945, 2015/07) | Greenstar 9i, 12i, 15i, 18i, 21i and 24i System ErP, natural gas and LPG | An internal error: "Control box or HCM is defective or loose" |
The Greenstar 8000 Life manual uses the same cause number, 233, for "Boiler identification module or device electronics faulty". That wording explains what the part does: it tells the main electronics which boiler they are fitted to. Without it, or with a poor connection to it, the control board will not run the boiler. Other Worcester Greenstar models that use cause codes may show the same fault, so check the manual for your exact boiler.
A locking fault
The System Compact manual lists 9U 233 in its locking fault table with the reset type given as the reset button. Worcester explains that a locking fault flashes a warning triangle with the code and needs someone to press reset on the boiler fascia. A blocking fault, by contrast, usually clears by itself once the cause has gone. So with 9U 233 the boiler stays off until it is reset and the fault has cleared.
Why 9U 233 appears
The causes in the manuals are short and all sit inside the boiler:
- The HCM or code plug is not connected or not pushed fully home.
- The HCM or code plug is faulty and the board cannot read it.
- The control box is loose or defective, or its connections are poor.
Because the first remedies are about checking the module is seated properly, the code is worth flagging if anyone has worked inside the boiler recently, for example to replace the control board or another part. Mention it even if the work seemed unrelated.
What you can safely do
- Note the full code, 9U 233, and your boiler model from the front panel or user manual. Do not let anyone talk about "U9" alone, as it will not match the tables.
- Check the fused spur feeding the boiler is switched on and the boiler has power. Note if the fault followed a power cut.
- Press the reset button on the boiler fascia, as your user manual describes. If 9U 233 comes straight back, try once more at most.
- If the boiler runs after a reset but the code keeps returning, record how often, as an intermittent connection behaves differently from a dead part.
Our Worcester boiler reset guide covers the reset method on different Greenstar models.
Do not remove the casing to reseat the HCM or code plug yourself. The control box carries mains voltage and sits beside the gas controls. Reseating, testing and replacing these parts is work for a Gas Safe registered engineer.
What a Gas Safe engineer will do
The manuals give engineers a clear order. They check the HCM is inserted properly, check the connections of the control box, replace the HCM if necessary and, if that does not clear the code, replace the control box or board. The System ErP manual also refers to checking the HCM's number, so the engineer will confirm the module matches your boiler before ordering parts.
When you book, say "9U, cause code 233" and give the model, such as Greenstar 30i System Compact or 24i System ErP. That lets the engineer bring the right module. If your display instead shows F0 263, see our page on that code, and our boiler PCB replacement cost guide explains what to expect if the board needs changing.
Typical repair costs in the UK
| Job | Typical UK cost |
|---|---|
| Engineer call-out and diagnosis | Typically £60 to £150 |
| Minor repair, such as reseating or replacing the plug-in module or a connection | Typically £100 to £180 |
| Control board (PCB) replacement, fitted | Typically £250 to £550 |
If the module just needs reseating, the bill may be little more than the call-out. A replacement control board is the expensive end, so ask the engineer to confirm the module and connections first.
Is it safe to keep trying the boiler?
9U 233 is an electronics fault, not a gas leak, and the lockout is the boiler protecting itself. The risk comes from repeated resets or from opening the casing. If you smell gas at any point, leave the boiler alone, turn off the gas at the meter if it is safe and call the National Gas Emergency Service on 0800 111 999.