F6
Glow-worm
Flow temperature sensor fault
F6 displayed, furnace cycles on and off rapidly or fails to modulate correctly
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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
Open-circuit or shorted flow NTC sensor, Loose sensor connection on Glow-worm PCB, Wiring damage in sensor harness, Sensor not properly clipped to flow pipe
How to Fix / Troubleshooting
Safety: Isolate power before removing the front panel. Surfaces may be hot.
Basic checks:
- Allow the furnace to cool and then reset it once. If F6 returns, further diagnosis is needed.
Technician steps:
- Locate the flow temperature NTC sensor on the flow pipe leaving the primary heat exchanger.
- Check the plug connection at the sensor and at the PCB for corrosion or loose pins; reseat connectors.
- Measure the resistance of the NTC at a known temperature and compare with Glow-worm resistance tables.
- Inspect the cable for damage or signs of overheating.
- Replace the flow NTC sensor if readings are out of range or intermittent.
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Repair Difficulty
Medium
3/5
Required Part
Flow Temperature NTC Sensor
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