Worcester Bosch b7 257 fault codewhat it means and how to fix it
An internal error in the burner control. Worcester's remedy is the control board, after checking the heat control module is seated. There is nothing to try beyond one reset.
Take care
Get a Gas Safe registered engineer to look at your Worcester Bosch b7 257
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What b7 257 means
b7 is the burner control code and it is published without much explanation on every range that has it. The Greenstar i ErP table gives "b7 257 — internal error — control box or the HCM is defective — check HCM is inserted properly, connections of the control box, contact technical helpline". The Compact family tables give "b7 257 — internal error — replace control board", with no reset type listed, which in Worcester's notation means the reset button is not expected to clear it.
The Highflow CDi manual, a two-character display, has "b7 — burner control error — replace control board". The burner control is the part of the board that sequences ignition and supervises the flame, and 257 is one of a run of internal self-check failures — 237 to 239, 242 to 256, 258, 259, 262, 263 — that Worcester publish mostly under F0 and, for this one number, under b7. What distinguishes it from the rest is only the letters on the display.
An engineer treats it the same way as the F0 internal errors: confirm the heat control module is pushed fully home and is the right one, confirm the board connectors are seated and dry, power down and up once, and if b7 returns, replace the board. It is a code that appears more often after moisture than after age. A leak from the hydraulics above the board, or condensation from a poorly sealed case, gets into connectors and produces internal errors that look random until the board is dried out or replaced.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| Control board fault | Worcester's stated remedy on every range that publishes b7. A board that fails its own burner-control checks is replaced rather than repaired. | Common |
| Heat control module not fully seated | On the Greenstar i the first check is that the HCM is pushed home; a module that is proud after a repair can produce internal errors across the F0 and b7 range. | Occasional |
| Moisture in the board connectors | Water from a weeping joint or condensation inside the case reaches the board and produces intermittent internal errors before it kills the board outright. | Occasional |
Can you fix it yourself?
Stop — this needs an engineer
What it costs to fix
Typical repair cost
£250–£450
Time on site
1 hour
Control board £250–£450 fitted, including transferring the heat control module.
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 b7 257
Is b7 the same as F0?
Practically, yes. Both are internal errors in the control, both are diagnosed by checking the module and connections and then replacing the board. Worcester simply publish cause code 257 under b7 rather than F0.
Why does the Compact manual not list a reset type for it?
Because Worcester do not expect the reset button to clear an internal control error. The row says replace control board and nothing else. A reset is harmless to try once, but the fix is the part.
How much is a control board?
On a Greenstar i or Compact, £250 to £450 fitted, and the engineer will transfer or replace the heat control module at the same time.
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.