F11
Glow-worm
Return Temperature Sensor Fault (NTC Open/Short)
Boiler displays F11, may run briefly then shut down, flow temperature reading erratic
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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
Failed return NTC sensor, Loose NTC connector, Damaged wiring harness, PCB sensor input fault
How to Fix / Troubleshooting
Safety: Turn off power before removing covers. Allow system to cool before touching pipework.
- Reset and observe: Reset the boiler. If F11 returns quickly, suspect the return NTC or wiring.
- Locate return NTC: The return temperature sensor is usually on the return pipe entering the boiler. Check that the connector is fully seated and free from corrosion.
- Check for moisture: Look for signs of water ingress around the sensor or wiring that could cause shorting.
- Engineer tasks:
- Measure NTC resistance and compare to specification.
- Check continuity of the sensor cable to the PCB.
- Replace the return NTC if readings are incorrect.
Do not bypass sensors; they are critical for safe boiler operation.
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Repair Difficulty
Medium
3/5
Required Part
Return NTC Temperature Sensor
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