Indoor and Outdoor Unit Communication Error

What it means

This code appears on LG inverter split air conditioners and on LG ducted and cassette units that use CH codes. It means the indoor board is no longer receiving a valid signal from the outdoor unit. The compressor stops, the indoor fan may keep running, and there is no cooling until communication is restored. The same code can mean something else on other brands.

Models covered: LG inverter split air conditioners (e.g. Dual Inverter) and LG ducted/cassette units that use CH codes; the code shows as CH plus a number on the display or as LED blinks.

Related codes: CH53CH23CH21CH01

Symptoms4

• CH05 appears a few minutes after start-up or straight away.
• The outdoor unit does not run at all (no compressor, no fan).
• The fault sometimes starts after a power cut, voltage fluctuation, or work on the wall or roof.
• The unit may run for a while, then stop with the code on an intermittent basis.

Possible causes6

• Loose, corroded or wrongly connected communication cable terminals at the indoor or outdoor terminal block.
• No power to the outdoor unit because its separate breaker is off or a fuse on its board has blown.
• Damaged interconnecting cable from sun, pests or being cut during building work.
• Outdoor inverter PCB fault that stops it from booting, sometimes caused by a failed IPM.
• Faulty communication circuit on the indoor PCB.
• Electrical noise, very low supply voltage or poor earthing.

How to diagnose6

1. Switch off the breaker for several minutes, restart, and make sure the outdoor unit breaker (if separate) is on.
2. Watch the outdoor unit after start-up: does the fan turn or a board LED light? No sign of life points to no power reaching it.
3. A technician checks the communication terminals at both units for correct order, tightness and corrosion.
4. Check continuity and insulation of each wire in the cable, and confirm power reaches the outdoor unit.
5. Check the outdoor board fuse and indicators; a blown fuse is usually the result of another fault, such as the IPM, not the root cause.
6. If wiring and power are good, find which board has failed by testing or by trying a known-good board.

How to fix6

1. Tighten, clean and correct the order of the communication terminals.
2. Repair or replace the damaged interconnecting cable with the correct size.
3. Restore power to the outdoor unit (breaker or fuse) after finding why the fuse blew.
4. Fit a voltage protector if power fluctuations are frequent.
5. Replace the failed PCB (usually the outdoor one, sometimes the indoor one) based on the tests.
6. Run the unit and confirm the outdoor unit starts and the code does not return after a long run.

The cost is low when the cause is a loose terminal or a damaged cable, and much higher when the outdoor inverter PCB has to be replaced.

Prices are rough estimates including the part and the technician’s fee, and vary with brand, model and part source.
Prices last updated: September 2026

Frequently asked questions