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Air Conditioner Indoor-outdoor communication cable: What It Does and Signs It's Failing

On most mini-splits, the cable between the indoor and outdoor units carries both power and the data signal the boards use to talk. LG notes that some of its newer systems power each unit separately. Wired wall controllers have their own thin cable to the indoor unit.

How it works

The boards exchange status messages over the cable constantly. If the messages stop or get garbled, the units stop and show a communication code.

Signs it is failing

• Communication codes right after install or a storm
• Indoor unit on but outdoor unit idle
• Codes that come and go
• Chewed or damaged cable outdoors

Brands list loose terminals, damaged or miswired cable, outdoor power switched off and bad boards as causes. Daikin adds address-setting mismatches on larger systems.

Can you replace it yourself?

Check that the outdoor disconnect and breaker are on, then reset power to both units. The cable carries line voltage on most systems, so inspecting terminals and replacing cable is electrician or HVAC work. Fujitsu advises against splices and wire nuts.

Buying the right part

Use the cable type the installation manual specifies. A tech should run it.

Error codes linked to the indoor-outdoor communication cable

• U4Indoor and outdoor units can't communicateDaikin
• 11Indoor and outdoor units can't communicateFujitsu
• 12Wired remote controller communication errorFujitsu
• E6Indoor and outdoor units can't communicateGree
• CH05Indoor and outdoor units aren't communicatingLG
• CH03Wired wall controller problemLG
• CH93Outdoor unit not powered or not respondingLG
• EL 01Indoor and outdoor units have lost contactMidea
• E0-E5Wall controller communication errorMitsubishi Electric
• E6 / E7 / E9Indoor and outdoor units aren't communicatingMitsubishi Electric
• E101Indoor and outdoor units aren't communicatingSamsung

Last reviewed 2026-10-19.