C6
Junkers
Fan speed control / feedback error
Boiler shows C6; fan runs erratically or at incorrect speed
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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
Defective fan speed sensor, Faulty fan motor, Faulty main PCB fan driver, Blocked flue causing abnormal load
How to Fix / Troubleshooting
Safety: Turn off power and gas before removing the combustion cover.
- Check flue and air intake: Ensure the concentric flue is not blocked and that the air intake is clear.
- Listen to fan: On start-up, listen for grinding, rattling, or intermittent fan operation, which indicates mechanical failure.
- Technician actions:
- Measure fan supply voltage and speed feedback signal according to Junkers service data.
- Inspect the fan connector and harness for loose pins or heat damage.
- Replace the fan assembly if speed cannot be controlled or feedback is missing.
- If a known-good fan still triggers C6, replace or repair the main PCB.
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Repair Difficulty
Hard
4/5
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