Worcester Bosch F1 fault codewhat it means and how to fix it
An internal data error in the control board — Worcester call it a ROM error on the Highflow. The board's own memory check has failed and the remedy is the board itself.
Take care
Get a Gas Safe registered engineer to look at your Worcester Bosch F1
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.
What F1 means
F1 is the sibling of F0 that Worcester reserve for the control's memory. The Highflow CDi manual is the plainest: "F1 — ROM error check — check all electrical connections, replace control board as necessary". The 2014 Greenstar 24/28/30 gives "F1 — internal data error — see servicing instructions for heating contractors", and its neighbours b2 and b3 in the same table are the same kind of thing. The Greenstar i ErP table publishes F1 with two cause codes, 259 and 263, both as "internal error — control box or the HCM is defective — check HCM is inserted properly, connections of the control box, contact technical helpline".
So on every range F1 means the board has failed a check on its own stored data. That can be the firmware, the calibration values, or the settings copied from the heat control module or code plug. It is not a fault in anything you can see or hear — the fan, the pump and the gas valve are all fine — and it does not respond to the usual householder actions.
The engineer's sequence is short: confirm the module or plug is seated and correct, power the boiler fully off and on, and if F1 comes back, change the board. Where it appears alongside 235 in the Greenstar i history — "incompatible HCM software version" — the cause is a module and board from different software generations, and Worcester's remedy is a board with the latest software rather than a different module.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| Control board memory fault | The board's stored program or data has become corrupt, and its self-check refuses to run the appliance on it. Worcester's remedy is replacement. | Common |
| Module and board software mismatch | On the Greenstar i a heat control module newer than the board it is plugged into gives internal errors; Worcester log 235 for this and call for a board with current software. | Occasional |
| Electrical disturbance | A supply spike or a nearby lightning strike can corrupt stored data. A single occurrence that clears on a power cycle is treated as this; a recurrence is the board. | Occasional |
Can you fix it yourself?
Stop — this needs an engineer
What it costs to fix
Typical repair cost
£0–£450
Time on site
1 hour
Free if a power cycle clears it. Control board £250–£450 fitted.
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
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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 F1
How is F1 different from F0?
F0 is Worcester's general internal error and covers most of the board's self-check failures. F1 is specifically the memory or data check. In practice both end with the same visit and, if they persist, the same part.
What are b2 and b3?
Data error codes on the 2014 Greenstar 24/28/30 and the Highflow, listed alongside F1 and with the same remedy. Worcester publish them without further description.
Will the boiler run again after a power cycle?
Sometimes, if the error was a one-off. If it does, watch it for a few days. A board whose data has genuinely corrupted will keep failing the check, and a replacement is the only lasting fix.
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.