S1
Junkers
Chimney / Flue Sensor or Draft Fault (on models with flue sensor)
S1 displayed, furnace in standby, does not fire even though heat is requested
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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
Blocked flue, Faulty flue gas temperature sensor, Incorrect flue installation, Sensor wiring fault
How to Fix / Troubleshooting
Safety first: Do not operate the furnace with a suspected flue fault. Risk of carbon monoxide.
- 1. Inspect flue terminal: Check outside terminal for obstructions. Clear any debris.
- 2. Flue sensor check: Locate the flue gas temperature sensor on the flue hood or pipe. Inspect for damage and secure mounting. Test resistance and replace if out of specification.
- 3. Wiring: Check sensor wiring for continuity and secure connections to the PCB.
- 4. Professional flue inspection: Have a qualified technician inspect the full flue route for blockages, incorrect slopes, or disconnections.
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Repair Difficulty
Hard
4/5
Required Part
Flue Gas Temperature Sensor
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