C5
Viessmann
Outdoor Discharge Pipe Temperature Sensor Fault
Outdoor unit stops unexpectedly; C5 error logged; may occur during defrost cycles
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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 or shorted discharge temperature sensor, Sensor detached from discharge line, Damaged sensor harness, Faulty outdoor PCB
How to Fix / Troubleshooting
Safety: Isolate power at the outdoor disconnect. The discharge line can be very hot during operation.
- Locate discharge sensor: Find the sensor clipped to the small-diameter copper pipe exiting the compressor (discharge line).
- Check attachment: Ensure the sensor is firmly clamped to the pipe with thermal paste or tape as originally installed.
- Inspect cable: Look for heat damage, brittle insulation, or rodent bites along the sensor cable.
- Test sensor: Disconnect and measure resistance at ambient temperature. Replace if readings are open, shorted, or far from spec.
Persistent C5 with a good sensor indicates a likely outdoor PCB fault requiring replacement.
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Repair Difficulty
Hard
4/5
Required Part
Discharge Pipe Temperature Sensor
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