The fault code F00 on a Vaillant boiler signals a problem with the heating‑flow temperature sensor (NTC thermistor). Essentially, the boiler has detected an interruption or fault in the heating‑flow sensor path, meaning the component cannot correctly monitor the water temperature of the heating circuit. This fault will cause the boiler to lock out as a safeguard, suspending heating operation until the fault is addressed.
Below are typical reasons why the F00 fault code appears:
Because this fault relates to internal sensors and wiring, only a Gas Safe registered engineer should perform the actual repair work. However, as a homeowner you can check the following:
Ensure boiler power is off at the isolator before any checks.
Confirm that the NTC thermistor plug appears seated and not visibly damaged.
If accessible, note any warning messages or display codes; then contact a qualified engineer to inspect the sensor, wiring harness and any related PCB connections.
| Cause | Description | Recommended Action |
|---|---|---|
| Flow NTC Thermistor Fault | The sensor monitoring heating‑flow temperature is defective or not reading. | Engage a Gas Safe engineer to test/replace the thermistor. |
| Loose or Disconnected Plug | The sensor plug or wiring loom has come loose or been disconnected. | Engineer to reseat or replace plug and inspect wiring loom. |
| Wiring Harness Fault | Interruption, short or damage in the wiring between NTC and PCB. | Qualified engineer to trace and repair harness and test continuity. |
While you can attempt a reset of the boiler (consult your manual and switch the power off for a minute before powering back on), if the F00 persists the root cause remains and must be addressed by a professional.
Schedule annual boiler servicing to check sensors, wiring and overall system health.
Monitor your system’s heating flow behaviour – unusually long heating times or inconsistent temperatures may hint at sensor issues.
Consider enrolling in a home‑care plan which covers regular servicing and helps keep your system under warranty.
Regular boiler servicing, prompt repairs and the right cover plan keep your heating system running smoothly. For example, our services include expert boiler installations (including combi boiler conversion), smart thermostat installations and home‑care cover.
If your Vaillant boiler displays fault code F00, it indicates a heating‑flow sensor (NTC thermistor) fault, wiring issue or connection problem. While you can reset the boiler, a persistent F00 requires intervention from a Gas Safe registered engineer to safely restore operation and maintain system warranty.
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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