Worcester Bosch A0 809 fault codewhat it means and how to fix it
The sensor reading the Heatslave II's internal hot water store has failed or lost its connection, so the boiler cannot tell how hot the store is and locks out until it is checked or replaced.
Take care
Get an OFTEC registered technician to look at your Worcester Bosch A0 809
We do not pass enquiries to a repair firm, so there is no form here. Find an OFTEC registered technician through the OFTEC, and ask to see their registration card when they arrive — it is the register, not the van, that tells you they are qualified.
Is there any hot water?
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 and no heating, blue light flashing
The boiler is in lock-out and nothing runs until the sensor fault is cleared.
Press reset once. If A0 809 returns, book an OFTEC registered technician and quote the code — it is usually a quick sensor swap.
It reset and has been fine since
An intermittent connection at the sensor or its plug rather than a dead sensor.
Note the date and mention it at the next service so the connector is checked before it fails on a cold night.
Hot water had been erratic — very hot, then cold — for a while before the code
A sensor drifting before it failed outright. The store was being run to the wrong temperature on its readings.
Still the sensor. Tell the technician about the run-up: it saves them looking at the diverter valve first.
What A0 809 means
The Heatslave II does its hot water differently from a wall-hung combi. Instead of heating mains water on demand alone, it keeps an internal thermal store hot and draws on it, and the info menu shows the store's set point — chosen with the hot water knob — and its current temperature. A0 809 is the sensor that reads that store failing. Table 12 of the 2022+ manual gives it as "thermal store sensor defective — reset — check and replace defect sensor as necessary", which is as much as Worcester say, and the reasoning is straightforward: with the sensor open or shorted the controls cannot know when the store needs reheating or when to stop, so they lock out rather than guess.
Three sensors sit on the hot water side of this boiler, and it helps to keep them apart. A7 808 is the hot water outlet sensor; H27 is the sensor on the incoming cold mains, which Worcester say does not affect operation when it fails; and A0 809 is the store sensor, the one that stops the boiler.
The same sensor is also one of the three the appliance frost protection watches — Worcester list the internal tank sensor alongside the primary heat exchanger and flow sensors — so with it defective the boiler cannot protect the store from frost either, which is a reason not to leave the fault over a cold spell. The operating instructions allow one press of the reset button for any flashing code, and a sensor that has genuinely failed will bring the code straight back. A sensor that has merely been disturbed at its connector may not, which is the one useful thing a reset tells you.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| Sensor failed open or short circuit | A thermistor that has drifted or failed reads as a temperature that cannot exist, and Worcester's only published remedy is to check and replace it. | Common |
| Connector or lead off the sensor | The sensor plugs into the low-voltage terminal strip in the control box. A connector disturbed during a service, or a lead chafed where it runs to the store, reads exactly like a defective sensor. | Common |
| Control board input | Where a new sensor reads correctly at its plug and the code stays, the board's input is the remaining suspect. It is the last thing to condemn, not the first. | Rare |
Can you fix it yourself?
Stop — this needs an engineer
What it costs to fix
Typical repair cost
£60–£220
Time on site
1 hour
Mostly the visit. Call-out £60–£120, the sensor itself £30–£80 fitted. A control board is only in question when the sensor and its wiring both test good, and it would be several hundred pounds.
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 cylinder / heat bank temperature sensor (tank NTC)
£15–£45Authorised Worcester parts distributor
Price checked 28 September 2026
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Related Worcester Bosch fault codes
- E2 233Get it checkedThe heat control module, the code plug that tells the board which Heatslave II it is running, is missing, loose or faulty. 9A 360 is its sibling: a plug fitted for the wrong boiler.Related: oil sensors and code plug
- H27Usually simple to fixA warning triangle with H27 means the sensor on the incoming cold mains has failed. Worcester say operation is not affected, so heating and hot water carry on until the sensor is replaced.Related: oil sensors and code plug
- 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 A0 809
What is the thermal store?
A tank of heated primary water inside the boiler that the mains water draws heat from when a tap runs. The Heatslave II's info menu shows its set point, selected with the hot water knob, and its current temperature. It is what lets a floor-standing oil combi deliver hot water at a rate a wall-hung boiler of the same output could not.
Is A0 809 the same as A7 808?
No. Both are hot water sensors and both are reset codes, but 808 is the hot water outlet sensor and 809 is the store sensor. The store sensor is the one the controls need to run the boiler at all, which is why this code locks it out.
Can I run the heating with the sensor faulty?
Not while the code is showing — the lock-out stops both heating and hot water. And because the store sensor is one of the three the frost protection reads, a boiler left off with this fault over a cold spell is a boiler that cannot protect its own store from freezing.
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.