LG S12ET NSJ Dualcool Smart: CH05 Error Code and Diagnosis

⚠️ SAFETY WARNING / SERIOUS RISK. This appliance operates with critically dangerous components (High Voltage or Gas). Before any inspection, it is essential to disconnect the electrical power by acting on the dedicated general miniature circuit breaker. DIY repair of critical devices can be fatal and is prohibited by law. Physical intervention legally requires qualified specialized technicians to issue compliant safety certificates. ReeFix provides this diagnosis EXCLUSIVELY for educational and informational purposes.

CAUSE A: Deteriorated or non-compliant interconnection wiring

Probability: 45%

This is the most frequent cause for the CH05 error. Serial communication between the indoor and outdoor units (terminal 3, "S" or "Data") is sensitive to interruptions, oxidation, or electrical noise. An undersized cable (less than 1.5 mm²), inadequate splices, or oxidized contacts increase resistance, preventing data exchange. The lack of efficient grounding amplifies disturbances.

• Key signals/indicators:
  • Presence of splices on the interconnection cable.
  • Loose or corroded terminals inside the terminal blocks (accessible only with the panel open).
  • The indoor unit turns on but the outdoor unit shows no signs of life, or the compressor does not start.
• Quick checks (by technician):
  1. Inspection of the wiring between the two units, checking for mechanical damage or improvised splices.
  2. Checking the tightening and cleanliness of the terminals on the terminal blocks of both units.
  3. Measurement of the electrical continuity of the communication conductor (terminal 3) and other cables.
• Necessary tools (for technician):
  • Professional True RMS Digital Multimeter for continuity tests.
  • Dry Electrical Contact Spray for terminal cleaning.
• Experience signal: This often happens on dated installations or those performed with non-professional materials, where exposure to atmospheric agents has deteriorated the insulation or oxidized the contacts. A typical mistake is underestimating the importance of shielded cable and proper grounding.
• Output for technician: Verify physical integrity and continuity of the interconnection cable (especially conductor 3 and ground). Clean and tighten terminals. Consider replacing with FG16OR16 4G1.5 Electrical Cable.

CAUSE B: Outdoor unit electronic board failure

Probability: 35%

If the outdoor unit's logic board does not receive power or its communication circuits are damaged, it cannot respond to the indoor unit, generating CH05. This can result from a blown fuse, a PFC reactor failure, or damage to the optocouplers on the board.

• Key signals/indicators:
  • The outdoor unit is completely inert: no diagnostic LEDs lit on the PCB (if present).
  • Total absence of noise from the outdoor unit (fan, compressor).
  • The voltage on the serial line (terminal 3) measured by the technician is fixed at 0V DC.
• Counter-examples: If the outdoor unit shows some signs of life (e.g., the power LED is on, a slight hum is heard), a total absence of power on the board is less likely.
• Quick checks (by technician):
  1. Check for 230V AC power on the L and N terminals of the outdoor unit.
  2. Inspect the fuses on the LG Dualcool Outdoor Board.
  3. Measure the low voltages (5V, 12V, 15V DC) at the board's test points.
• Experience signal: A transient overvoltage, such as a nearby lightning strike or significant power grid fluctuations, can easily damage sensitive components of the outdoor board, particularly optocouplers or power supply circuits.
• Output for technician: Check for 230V AC power on the outdoor unit. Check the integrity of the fuses on the outdoor PCB and the secondary power supply voltages. Analyze the serial transmission circuits.

CAUSE C: Indoor unit electronic board failure

Probability: 15%

Less likely than the previous ones, but possible. If the serial transceiver circuit on the LG Dualcool Indoor Board is faulty, the indoor unit cannot send or receive data packets, even if the outdoor unit is perfectly functional and the wiring is intact.

• Key signals/indicators:
  • The indoor unit turns on regularly, responds to remote control commands, but the outdoor unit remains off.
  • The voltage on the communication terminal (3) measured by the technician remains fixed at the maximum value (approximately 70-80V DC) without any oscillation.
• Quick checks (by technician):
  1. Measurement of DC voltage on terminal 3 of the indoor unit.
  2. Checking components on the indoor board for signs of burns or swelling.
• Mini-glossary:
  • Optocoupler: An electronic component that allows the transmission of an electrical signal between two galvanically isolated circuits, using light. Essential for safety and data communication.
• Output for technician: Perform a dynamic signal analysis on the serial line of terminal 3 of the indoor unit. Verify the integrity of the optocouplers dedicated to communication.

WHICH IS YOURS?

The most probable cause (45%) for the CH05 error on your LG S12ET NSJ Dualcool Smart air conditioner is a problem related to the interconnection wiring or serial communication between the units. Secondly (35%), the fault could lie in the outdoor unit's electronic board, while an anomaly in the indoor unit's board is decidedly less frequent (15%).

Given the complexity of the diagnosis, the risks associated with high voltage and the presence of refrigerant gas, it is imperative to contact a qualified technician to perform instrumental checks safely and avoid irreparable damage to electronic components.

Many technicians on industry forums confirm that a blown fuse on the outdoor board is the main suspect when the unit shows no signs of life. Replacing this small component can often resolve the CH05 error without having to replace the entire electronic board.