What CH05 means

LG split ACs use a communication cable (typically a 3-core or 4-core cable) between the indoor and outdoor units. This cable carries both power and a data signal. When the signal is interrupted — by a loose connection, wrong wiring, or a failed PCB — the indoor unit displays CH05 within about five minutes of power-up.

On the outdoor unit, you can confirm the issue by checking the PCB LEDs: the green LED should be steady (power present), and the red LED should be flickering (receiving data from the indoor unit). If the red LED is steady (not flickering), no data is being received.

Note: CH05 is the most common LG AC error code. In most cases, it is a wiring problem — not a board failure.

Step-by-step troubleshooting

  1. Power off both units at their respective breakers.
  2. Check the communication cable at both ends. At the indoor unit terminal block and at the outdoor unit terminal block, ensure each wire is in the correct terminal (match the labels: S, 1, 2, N or similar).
  3. Check for switches or relays on the cable. Some installations route the comms cable through a separate isolator. Ensure it is on and passing the signal through.
  4. Remove external devices. Condensate pumps, timers, or other third-party devices connected to the communication cable can interfere. Disconnect them temporarily to test.
  5. Measure voltage. Between terminals N and 3 (signal), you should see 0-65 Vdc fluctuating. A steady 0V means no signal.
  6. Power both units on and wait five minutes. Check the outdoor unit red LED — it should flicker if communication is restored.
  7. If CH05 persists with confirmed good wiring, the indoor or outdoor PCB may be faulty. A technician can swap-test boards to identify which one.