Viessmann Outdoor Unit Not Responding

Indoor unit fan runs but outdoor unit does not start; no error code shown

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.

Possible Causes

Outdoor isolator switched off, Tripped outdoor fuse, Broken communication or power cable to outdoor unit, Faulty outdoor PCB

Safety: Turn off power before opening the outdoor unit.

Check outdoor isolator: Ensure the external disconnect switch is ON.
Inspect fuses: Check any fuses in the outdoor unit and replace with identical types if blown.
Verify power: Measure voltage at the outdoor terminals to confirm supply.
Check interconnect cable: Inspect for damage and verify correct wiring between indoor and outdoor units.

If power and wiring are correct but the outdoor unit remains dead, the outdoor PCB or compressor contactor (if fitted) may need replacement.

Hard 4/5

Outdoor Main Control PCB

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