The Vaillant F01 fault code indicates a problem with the heating return temperature sensor, also known as the return NTC thermistor. This sensor is responsible for monitoring the temperature of the water returning to the boiler. When it stops reading correctly or becomes disconnected, the boiler immediately shuts down to prevent unsafe operation.
This guide explains what the Vaillant F01 error means, the common causes, how an engineer fixes it, repair costs, and preventative advice.
When the Vaillant boiler displays F01, it means the return NTC sensor is reading “open circuit” or is giving a reading outside the expected range. In simple terms, the boiler cannot correctly detect the temperature of the water coming back into the system.
Because accurate temperature readings are essential for safe operation, the boiler will lock out and stop heating until the fault is resolved.
A-rated boilers installed from £1795
Below are the typical causes of this fault. A Gas Safe engineer will confirm the exact reason:
Faulty return NTC thermistor
Loose or disconnected wiring harness
Corroded or wet electrical terminals
Damaged wiring between the sensor and PCB
PCB fault preventing correct temperature reading
Disturbed wiring after recent repair or maintenance
These issues require specialist testing and should not be handled by the homeowner.
| Cause | Description | Recommended Action |
|---|---|---|
| Faulty Return NTC Thermistor | The sensor is reading open circuit or outside expected resistance range. | Engineer to test and replace the NTC thermistor. |
| Loose or Disconnected Wiring | Wiring harness not connected properly or cable damaged. | Engineer to inspect, repair or replace wiring loom. |
| Corroded/Wet Connectors | Moisture or corrosion at the thermistor terminals prevents correct reading. | Dry, clean or replace connectors; inspect for system leaks. |
| PCB Fault | The PCB is unable to read the sensor correctly due to internal issues. | Engineer to test PCB and replace if required. |
You cannot repair this fault without removing the boiler casing — and doing so is illegal unless you are Gas Safe registered.
However, you can safely check:
The boiler has power
There are no visible leaks around the boiler
The boiler hasn’t tripped a fuse
If the code persists, a qualified engineer is required.
Get complete boiler cover from £12p/m
A Gas Safe engineer will:
Test the resistance of the return NTC thermistor
Inspect the wiring harness for breaks or loose plugs
Check for moisture or corrosion
Ensure the PCB is receiving correct readings
Replace the thermistor or wiring as needed
Reset and test the boiler under operating conditions
This is usually a straightforward repair.
Boiler not working? We’ll fix it from £108
| Repair Type | Typical Cost (UK) |
|---|---|
| Return NTC Thermistor Replacement | £90 – £160 |
| Wiring Harness Repair/Replacement | £100 – £180 |
| PCB Replacement | £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:
Get complete boiler cover from £12p/m
A-rated boilers installed from £1795
Book your boiler service for just £108
Boiler not working? We’ll fix it from £108
Premium cylinders installed from £1599
Free survey & quote for all ASHP installs
