C4
Viessmann
Outdoor Coil Temperature Sensor Fault
Unit runs but performance is unstable; C4 appears 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
Defective outdoor heat exchanger sensor, Sensor wiring damaged by weather or rodents, Corroded connector at outdoor PCB, Faulty outdoor main control PCB
How to Fix / Troubleshooting
Safety: Disconnect power at the outdoor isolator before opening the outdoor unit. Beware of sharp metal edges.
- Locate sensor: Remove the outdoor unit top or side panel. The coil sensor is clipped to the outdoor heat exchanger tubing or fins.
- Inspect physically: Check for cracked sensor body, UV-damaged insulation, or loose mounting clip.
- Check wiring and connector: Follow the sensor cable to the PCB. Re-seat the connector and inspect for corrosion or water ingress.
- Measure resistance: With power off, disconnect the sensor and measure resistance at ambient temperature. Compare to Viessmann NTC specifications. Replace if out of range.
If a new sensor does not clear C4, the outdoor PCB may be misreading the sensor and should be evaluated by a technician.
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Repair Difficulty
Medium
3/5
Required Part
Outdoor Coil Temperature Sensor (NTC)
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