If your boiler is showing the Vaillant F03 fault code, this refers to a problem with the cylinder sensor (actoSTOR NTC) – a temperature sensor used in boiler models with a separate hot‑water cylinder. When this sensor fails or its wiring is compromised, the boiler triggers the F03 error and locks out to prevent unsafe operation.
In this article you’ll learn what causes the F03 error, what a professional engineer will do to repair it, typical cost ranges, and how to avoid it in future.
The code indicates an interruption or fault in the cylinder sensor (actoSTOR NTC) or its wiring harness.
Essentially, the boiler cannot get a valid temperature reading from the cylinder sensor, which is critical for safe hot water or heating operation.
Because the boiler cannot confirm correct water temperature / sensor data, it triggers a safety lock‑out – displayed as “F03” – and stops firing until the issue is resolved.
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The actoSTOR NTC sensor itself is defective or failed
The sensor wiring harness or plug is damaged, disconnected, corroded or loose
The cylinder sensor plug connection to the PCB or control module is faulty
Moisture, corrosion or mechanical damage around the sensor or wiring area
Previous maintenance or installation work caused an unintended wiring issue
Because of safety and legal restrictions, these checks and repairs must only be carried out by a Gas Safe registered engineer.
| Cause | Description | Recommended Action |
|---|---|---|
| Faulty cylinder (actoSTOR) NTC sensor | The sensor fails to register correct temperature or returns an open circuit. | Engineer to test sensor resistance and replace NTC if required. |
| Loose / disconnected wiring harness or plug | Wiring between sensor and control board is damaged or not connected correctly. | Engineer to inspect, re‑secure or replace wiring harness / plug. |
| Corroded or wet sensor connectors | Moisture or corrosion interrupts electrical signal to sensor. | Dry, clean or replace connectors; check for water leaks or condensation. |
| Faulty PCB / control electronics | Control board fails to read sensor data correctly due to internal fault. | Engineer to test PCB and replace if necessary. |
Because of legal and safety regulations:
Do not remove the boiler casing.
Do not attempt wiring checks or component replacement if you are not Gas Safe registered.
You can safely:
Check boiler has power and isn’t subject to leaks
Perform a basic reset to see if the code clears (only a test – issue likely remains)
If F03 returns, you must call a qualified engineer.
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A Gas Safe engineer will typically:
Because of its internal complexity, this fault is usually resolved in one visit if sensor or wiring is at fault.
| Repair Item | Estimated Cost (UK Typical Rates) |
|---|---|
| actoSTOR NTC sensor replacement | £90 – £160 |
| Wiring harness / plug repair or replacement | £100 – £180 |
| PCB / control board replacement (if needed) | £250 – £450 |
| Diagnostic call‑out (if sensor not replaced) | £70 – £120 |
(Estimates based on typical UK regional labour charges – actual costs may vary depending on boiler model and region.)
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| Repair Item | Estimated Cost (UK Typical Rates) |
|---|---|
| actoSTOR NTC sensor replacement | £90 – £160 |
| Wiring harness / plug repair or replacement | £100 – £180 |
| PCB / control board replacement (if needed) | £250 – £450 |
| Diagnostic call‑out (if sensor not replaced) | £70 – £120 |
To reduce the chance of the error returning:
Keep the boiler area dry and leak‑free
Use quality parts when repairs are needed
Consider a boiler cover plan for ongoing protection
Upgrade older sensors when advised
These steps help maintain accurate temperature control and prolong boiler lifespan.
For more information on other Vaillant fault codes, you can refer to the following:
Consider exploring the following services to maintain and enhance your heating system:
For more information on Vaillant Boilers you can refer to the following articles:
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