Worcester Bosch A7 fault codewhat it means and how to fix it
The hot water temperature sensor has failed. Heating usually carries on as normal while the hot water runs cold, scalding, or swings between the two.
Take care
Get a Gas Safe registered engineer to look at your Worcester Bosch A7
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 are the taps actually doing?
The right answer depends on what you can see. Pick the one that matches.
Choose the option on the left that matches what your appliance is doing, and the answer for that case appears here.
No hot water at all, heating fine
Classic A7. The boiler will not heat water it cannot measure, so it declines rather than guess.
Book a repair and use the heating in the meantime. Nothing about this is unsafe.
The water runs dangerously hot
A sensor reading low makes the burner keep going when it should have modulated down. Scald risk is real, particularly for children and older people.
Stop using that hot tap, or run it well mixed with cold, until it has been repaired.
It cycles between hot and cold in the shower
An intermittent signal. The boiler is chasing a reading that keeps jumping, which usually means a marginal connection rather than a fully dead sensor.
Ask for the leads and plug to be inspected as well as the sensor itself.
What A7 means
A7 means the domestic hot water NTC sensor is defective, or the leads running to it are. That sensor is how the boiler knows the temperature of the water it is sending to your taps, and it adjusts the burner constantly against that reading. Without a reliable signal the boiler cannot regulate hot water sensibly, so it either refuses to heat water at all or overshoots and undershoots the target.
The distinctive thing about A7 is what it leaves working. Central heating uses a different sensor and normally continues without complaint, which is why people report a boiler that warms the house perfectly but cannot produce a decent shower. That combination is a strong hint on its own, and it is worth saying when you book the visit.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| Failed hot water NTC sensor | These drift out of tolerance or fail open with age. It is the usual answer and the cheapest one. | Common |
| Damaged or wet sensor leads | The thin pair of wires to the sensor chafes where it passes the case, and a joint above the boiler can drip onto it. | Common |
| Corroded connector at the board | Green deposits on the plug raise resistance enough that the board reads the sensor as out of range. | Occasional |
| Control board input failure | Ruled in only after the sensor has been measured across its temperature range and found healthy. | Rare |
Can you fix it yourself?
Stop — this needs an engineer
What it costs to fix
Typical repair cost
£110–£550
Hot water sensor £110–£190 fitted. Wiring repair often within a call-out charge of £80–£150. Control board, if it comes to that, £350–£550.
Typical UK prices including parts and labour, updated September 2026. Your quote may differ.
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
- 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
- 2980Stop — call an engineerSuper lock-out. Five locking faults were reset inside fifteen minutes while the boiler saw a potentially safety-critical condition, so it has locked itself against further resets. Only an engineer can clear it.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 A7
Why does the heating still work?
Because it relies on a separate flow sensor. A7 is specific to the hot water side, so the boiler carries on heating radiators quite happily. That split is the most useful clue the code gives you.
Is it safe to keep using?
The heating side is fine. The hot water side is fine only if the water is not coming out too hot — an unregulated burner can push tap temperatures to scalding levels. If it is running hot, stop using it until it is fixed.
How much should this cost?
This is one of the cheaper boiler repairs. Expect £110 to £190 for the sensor and labour together. If a wiring repair is all that is needed, a call-out charge may cover it.
Does the three-digit suffix matter here?
Less than usual. A7 is commonly listed without one, and where a suffix does appear it still points at the same hot water sensor circuit. Quote it if your display shows it, but do not worry if it does not.
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
- Manufacturer support portal
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.