Vaillant
Control Signal Failure to Outdoor Unit
Unit powers on but compressor and outdoor unit do not start, only indoor fan runs
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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
Broken communication cable between indoor and outdoor, Mis‑wired terminal connections, Faulty indoor PCB communication driver, Faulty outdoor PCB
How to Fix / Troubleshooting
Safety: Isolate power at both indoor and outdoor disconnects before working on wiring.
- Verify wiring: Open both indoor and outdoor terminal covers and confirm that the interconnecting cable is correctly wired and firmly secured.
- Continuity test: With power off, test continuity of each conductor in the interconnecting cable. Replace the cable if any conductor is open.
- Check for damage: Inspect the cable along its route for mechanical damage or water ingress.
- Power cycle: Restore power and test. If the outdoor unit still does not respond, the indoor or outdoor PCB may be defective and should be checked by a Vaillant technician.
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Repair Difficulty
Hard
4/5
Required Part
Indoor–Outdoor Communication Cable
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