F.11
Vaillant
Return Temperature Sensor Short Circuit
Boiler displays F.11 and locks out
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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 at return sensor, Shorted wiring harness, Defective main PCB
How to Fix / Troubleshooting
Safety: Turn off electrical supply and isolate gas. Wait for components to cool.
- Reset boiler: Attempt a reset from the control panel. If F.11 reappears, continue.
- Inspect for leaks: Check around the return pipe and sensor for any signs of water leaks from joints or valves. Repair leaks before replacing sensors.
- Check sensor and cable: Disconnect the return NTC and inspect the connector. Look for corrosion or bridging between pins. Examine the cable for crushed or melted sections that could cause a short.
- Measure resistance: Use a multimeter to test the NTC. A near-zero resistance indicates a shorted sensor; replace the return NTC.
- PCB diagnosis: If a new sensor and good wiring still give F.11, the PCB may be faulty. Replacement should be carried out by a Gas Safe engineer.
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Repair Difficulty
Medium
3/5
Required Part
Return NTC Temperature Sensor
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