Boiler/furnace locks out with F.01, runs briefly then shuts down on overheat
Safety Warning
This repair may involve working with high voltage components or water connections. Always unplug the appliance before removing any panels.
If you are not confident in your ability to perform this repair safely, we strongly recommend contacting a professional technician.
Possible Causes
How to Fix / Troubleshooting
Safety: Isolate electrical power and close the gas valve before working on the appliance.
Diagnostic steps:
- Locate the flow NTC on the flow pipe leaving the main heat exchanger.
- Inspect the sensor and cable for signs of water ingress, melted insulation, or rubbing against metal edges.
- Disconnect the NTC and measure resistance with a multimeter. A reading near 0 Ω indicates a shorted sensor; replace it.
- Check the harness from the NTC to the PCB for crushed or chafed sections. Repair or replace the harness if insulation is damaged.
- Dry any moisture in the connector and ensure a tight fit when reconnecting.
- Restore power and reset the boiler according to the Vaillant user instructions.
Note: If the NTC and wiring are fine but F.01 persists, the main PCB may be misreading the sensor and should be checked by a qualified technician.
Repair Difficulty
Required Part
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