If your boiler is showing the Vaillant F02 fault code, it means there’s a problem with the flow temperature sensor – also known as the flow NTC thermistor. This is the sensor that monitors the temperature of the water leaving the boiler to heat your radiators or hot water cylinder.
When this component fails or gives an invalid reading, the boiler locks out to prevent unsafe operation. In this post, we’ll explain what the fault means, the common causes, how to fix it, and when you should call a professional.
The Vaillant F02 fault code appears when the boiler’s flow sensor is either not sending a signal, reading incorrectly, or has become disconnected.
The flow sensor is a small but important part inside the boiler that constantly monitors the temperature of the water as it exits the boiler. If the boiler can’t get a reliable reading, it shuts down to prevent overheating or damage to the system.
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Below are the most common reasons for the F02 fault code. These issues can only be confirmed by a Gas Safe registered engineer:
Failed flow NTC thermistor
Loose or broken wiring to the flow sensor
Corroded or damp electrical connectors
Damaged wiring harness
Faulty PCB (printed circuit board)
| Cause | Description | Recommended Fix |
|---|---|---|
| Faulty Flow Thermistor | The flow sensor is no longer reading correctly or has failed completely. | Engineer to test and replace the thermistor if necessary. |
| Loose or Disconnected Wiring | Wiring between the sensor and PCB is damaged or unplugged. | Reconnect or replace wiring as needed. |
| Corroded Connectors | Moisture or corrosion at the plug causes signal loss. | Engineer to clean or replace the connectors. |
| PCB Fault | The main control board cannot read the flow sensor correctly. | Engineer to test and replace the PCB if faulty. |
No – this is not a DIY job. The flow temperature sensor is located inside the boiler’s sealed casing, and only Gas Safe engineers are legally allowed to remove that casing.
However, you can:
Check the boiler has power
Check for signs of leaks below the boiler
Reset the boiler once to see if the error clears (if it returns, call an engineer)
If the fault keeps returning, it needs professional attention.
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An experienced engineer will:
Test the flow sensor’s resistance using a multimeter
Check the wiring harness and plug
Inspect for moisture or corrosion
Confirm signal transmission to the PCB
Replace the sensor, connectors, or PCB as needed
Reset and test the boiler once repairs are complete
Most flow sensor replacements can be done in under an hour.
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| Repair | Estimated Cost |
|---|---|
| Flow Thermistor Replacement | £90 – £150 |
| Wiring Harness Repair | £100 – £180 |
| PCB Replacement (if needed) | £250 – £450 |
| Diagnostic Call-Out | £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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