Worcester Bosch b1 fault codewhat it means and how to fix it
The control board cannot find its code plug, the small module that tells it which boiler it is fitted to. Without it the board will not run. Usually follows a board or plug being disturbed.
Take care
Get a Gas Safe registered engineer to look at your Worcester Bosch b1
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What b1 means
Every Worcester Greenstar of the CDi Classic generation carries a code plug: a small keyed module pushed into the control board that identifies the appliance model and gas type, so that one board design can run a whole range. Worcester's table entry is short — "b1: code plug not detected — insert code plug correctly, test and replace if necessary" — and it appears in the CDi Classic ErP manual, the earlier CDi combi, system and regular manuals, the Highflow CDi manual and the 2014 Greenstar 24/28/30.
The engineer service booklet for the CDi Classic adds the procedure: read the plug number back through service function 8.b and compare it with the appendix, refit or replace the plug if the number is missing or wrong, and only if b1 persists with a correct, seated plug change the control board. The plug numbers are published per model and gas type, and the earlier CDi manuals print them in a table, so an engineer can confirm at a glance that the right one is fitted. The Greenstar i and the Compact ranges do the same job with a heat control module and report its absence differently, as 9U 233.
In practice b1 appears in three situations: a plug that has worked loose over years of thermal cycling, a plug knocked during a repair, and a plug missing altogether because a replacement board arrived without one and it was not transferred from the old board.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| Code plug not seated | The plug is a push fit into the board. Vibration and heat over years, or a knock during a service, can leave it a millimetre proud and unreadable. | Common |
| Plug not transferred to a new board | Replacement boards are supplied without the plug, which has to come across from the old one. b1 immediately after a board change is nearly always this. | Common |
| Wrong plug for the model | Worcester publish a different plug number for each output and gas type. A plug from a different model reads as invalid, and the board refuses to run rather than guess. | Occasional |
| Control board fault | If the correct plug is fitted, seated and reads back correctly and b1 remains, Worcester's booklet has the board changed as the last step. | Rare |
Can you fix it yourself?
Stop — this needs an engineer
What it costs to fix
Typical repair cost
£90–£450
Time on site
45 minutes
Reseating the plug £90–£140 for the visit. A replacement plug £120–£200 fitted. Control board £250–£450 if it is genuinely the board.
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
- 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
- 9U 233Get it checkedThe heat control module is loose, missing or faulty. This small plug-in part carries the identity of your boiler, and nothing runs without it.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
Own a Worcester Bosch boiler? Save it and its codes come first next time.
Related guides
Common questions about Worcester Bosch b1
What is a code plug?
A small module, about the size of a fingertip, that plugs into the control board and carries the identity of the boiler — model, output and gas type. Worcester call the equivalent part on the Greenstar i a heat control module. It lets one board serve many models, but it means the board is useless without it.
Is b1 the same as 9U 233 on a Greenstar i?
Functionally yes. Both mean the board cannot read the identification module. Worcester use b1 on the two-character displays of the CDi Classic era and 9U with cause code 233 on the Greenstar i and Compact ranges.
Can the engineer just fit any code plug?
No. The plug number has to match the model and gas type, and Worcester print the numbers in the installation instructions for each range. The right plug for a 30 CDi on natural gas is different from the one for LPG.
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.