F.11
Vaillant
Return Temperature Sensor Short Circuit
Furnace shows F.11, may overheat quickly or cycle rapidly
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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.
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Possible Causes
Shorted return NTC sensor, Water ingress in sensor plug, Harness short to ground or between conductors, PCB sensor circuit failure
How to Fix / Troubleshooting
Safety: Turn off power and allow the unit to cool.
Inspection steps:
- Access the return NTC on the return pipe.
- Check for moisture around the sensor and connector; dry thoroughly if present.
- Disconnect the NTC and measure resistance. A very low resistance (near 0 Ω) indicates a shorted sensor; replace it.
- Inspect the cable for crushed sections or insulation damage where it passes through grommets or clips.
- Repair or replace the harness if any conductor is exposed or shorted.
- Reconnect, restore power, and reset the boiler.
Warning: Do not bypass the NTC sensor; this is a safety device. If the fault persists with a new sensor, contact a professional to evaluate the PCB.
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Repair Difficulty
Medium
3/5
Required Part
Return NTC Temperature Sensor
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