C9
Viessmann
Outdoor Ambient Temperature Sensor Fault
Outdoor unit fails to start or stops; C9 error in diagnostics
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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 outdoor ambient sensor, Sensor exposed to direct sunlight or relocated, Broken sensor wiring, Faulty outdoor PCB
How to Fix / Troubleshooting
Safety: Turn off power at the outdoor isolator.
- Locate ambient sensor: The ambient sensor is usually mounted near the outdoor air intake grille, away from direct discharge air.
- Check mounting position: Ensure the sensor is not hanging loose or placed where it is heated by the sun or exhaust air.
- Inspect wiring: Look for mechanical damage or corrosion at the connector.
- Measure resistance: Test the sensor with a multimeter and compare to the NTC curve. Replace if defective.
If C9 remains after sensor replacement, the outdoor control board may be at fault.
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Repair Difficulty
Medium
3/5
Required Part
Outdoor Ambient Temperature Sensor
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