Worcester Bosch A6 fault codewhat it means and how to fix it
The boiler thinks an external limit thermostat has opened — usually the underfloor heating stat — or the link that stands in for one is missing. Nothing runs until that circuit is closed again.
Take care
Get a Gas Safe registered engineer to look at your Worcester Bosch A6
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 A6 means
Worcester's Heatronic boards have a pair of terminals, 8 and 9 on the ST8 connector, reserved for an external temperature limiter. On an underfloor heating system that is the stat that stops the floor being cooked if a mixing valve fails; on a plain radiator system nothing is connected and a wire link is fitted across the two terminals instead. A6 on a CDi Classic means the board sees that circuit open, and Worcester's service booklet reads it as the external underfloor heating thermostat defective, with the appliance not available for any demand.
On the Greenstar Highflow CDi and FS CDi the same fault is d3, external temp limiter, and the manual's remedy is blunt: connect terminals 8 and 9 on ST8. On the Compact ranges it is D3 with 232 beneath it, external cut-off switch activated. So the code has three spellings across three generations and one meaning.
In practice it appears in two situations. Either the underfloor stat has genuinely tripped or failed open, in which case the floor circuit needs looking at before the boiler; or somebody has been inside the boiler — a control board swap, a wiring tidy — and the link has come out or been left off. The second is the common one after a repair, and it is why Worcester's fault procedure starts with the question of whether anything is connected at ST8 at all before it touches the thermostat. It is an electrical interlock doing its job, not a combustion fault, and the boiler is not damaged by it.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| Link missing from ST8 terminals 8 and 9 | No underfloor thermostat is fitted, and the wire link that should bridge the terminals has been dislodged or left off after work on the board. Worcester's first step is to check whether anything is connected there. | Common |
| Underfloor heating limit thermostat tripped | The floor circuit has run too hot — usually a mixing valve or its actuator — and the stat has opened as designed. It will not close until the floor has cooled, and the fault behind it needs finding. | Common |
| Thermostat or its wiring open circuit | A failed stat or a broken lead reads the same as a tripped one. Worcester's procedure checks the thermostat for continuity, then the harness between it and the board. | Occasional |
| Control board input fault | If the link is fitted, the stat is closed and the harness is sound, Worcester's last step is the control board, transferring the service settings across. | Rare |
Can you fix it yourself?
Stop — this needs an engineer
What it costs to fix
Typical repair cost
£90–£250
Time on site
30 minutes
Refitting the link £90–£140 for the visit. Replacement underfloor limit thermostat £120–£250 fitted. A control board is the last resort and is unlikely to be needed.
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
Own this appliance and had this code? . Nothing is published until a person has read it.
Related Worcester Bosch fault codes
- A8Get it checkedThe boiler has lost communication with a Worcester control on the two-wire bus — a Sense, FX or FW controller, or a Wave thermostat. Heating usually carries on at the boiler's own settings.Related: electronics and communication
- H09Get it checkedThe control board has lost its data link to the pump. The boiler keeps running, but Worcester flag it because a pump the board cannot talk to is either wired wrongly or the wrong pump.Related: electronics and communication
- 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 A6
I have no underfloor heating. Why would this appear?
Because the terminals then need a link across them and it has come out. Worcester's remedy for the d3 version on the Highflow is literally connect terminals 8 and 9 on ST8. It usually follows a board swap or a wiring tidy.
Is A6 the same as d3 and D3 232?
Yes. A6 is the CDi Classic's spelling, d3 the Highflow and FS CDi's, and D3 232 the Compact ranges'. All three mean the external limiter circuit at ST8 is open.
Can I just reset it?
A reset will not clear it while the circuit is open. If the underfloor stat has tripped, wait for the floor to cool and it may close on its own; if the link is missing, nothing changes until it is refitted.
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.