LG Air Conditioner Error 40

⚡ Quick Answer: The LG air conditioner error code 40 indicates a communication failure between the indoor and outdoor units, resulting in the unit ceasing operation. ... ✅ DIY Fixable

What Does Error 40 Mean?

The LG air conditioner error code 40 indicates a communication failure between the indoor and outdoor units, resulting in the unit ceasing operation. This issue is often linked to a short-circuit condition in the current transformer (CT) circuit or related sensor connections, leading to a lack of normal cooling or heating functionality.

Common Symptoms:

  • Air conditioner stops operating when code 40 appears on display
  • Indoor or outdoor unit may not run
  • No normal cooling/heating operation

Possible Causes

  • CT (current transformer) circuit short or similar sensor/connection fault (low)

How to Fix Error 40

DIY Fix Power reset and basic checks

  1. Turn off the air conditioner using the remote and power switch
  2. Disconnect from mains power for 5–10 minutes
  3. Check that the outdoor unit is unobstructed and the indoor unit filter is clean
  4. Reconnect power and restart to see if the error clears

Technician Only Inspect CT/sensor circuits and connections

  1. Check current transformer (CT) and related sensor wiring for short circuits or damage
  2. Inspect indoor/outdoor communication and sensor harnesses
  3. Use diagnostic tools to read service codes and test circuit continuity
  4. Replace faulty CT or sensor modules as needed

Parts needed: Current transformer (CT) or sensor modules, Wiring harnesses/connectors

🛑 When to Call a Pro:
  • If the error persists after power resetting
  • If there are signs of electrical burning, smoke, or unusual noises
  • If the unit fails to restart or operates unsafely

Next step

Choose the next step for 40

Confirm the diagnosis before ordering. An error code can identify the system involved, but it does not prove that a specific part has failed.

Check during diagnosis

Most relevant component to inspect

Wiring Harness

Connects controls, sensors, motors, and other electrical components inside the appliance.

Typical part-only price
$20 - $120
DIY difficulty
Medium
Estimated time
30-90 mins
Where can I find the model number?

Check the rating label around the door, lid, frame, rear panel, or inside the appliance cabinet. Use the complete model number, including suffixes.

Find a part that fits my model

Verify the complete model and part number on the retailer's compatibility page before ordering.

Other parts options

Check related components first; use model search as a fallback.

Model number required

Model-compatible Air Conditioner parts

Enter the complete model number to browse compatible replacement parts. This is a model-based parts search, not a diagnosis that a specific component failed.

Find a part that fits my model

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Optional routine care

Maintenance supplies

These products support cleaning or routine upkeep. They are separate from the possible repair components above and are not claimed to fix this error code or symptom.

Maintenance supply

Air Conditioner Fin Cleaner

For routine cleaning of accessible coil fins when permitted by the manufacturer. Disconnect power and follow product and appliance instructions.

View maintenance supply
Maintenance supply

Model-Compatible Washable AC Filter

A reusable filter option only where the appliance supports one. Match the full model, dimensions, and filter specification.

Compatibility check required: use the complete appliance model and compare the existing filter, bag, or specification before ordering.

Find one that fits my model

Affiliate disclosure: As an Amazon Associate I earn from qualifying purchases. Recommendations are based on routine-care relevance, not commission rate.

Technical Specifications

wattage N/A
component Control Board
resistance N/A
part_number LG 6721A20001D
voltage_rating 220V AC

Common Questions

What does 40 error mean on LG air conditioner?

Error 40 signifies a communication failure between the indoor and outdoor units, preventing normal operation.

Can I fix LG air conditioner 40 error myself?

Yes, you can try resetting the unit and checking for obstructions or a clean filter.

How much does it cost to fix 40 error?

Repair costs range from ¥10,000 to ¥40,000+, depending on the model and region.

When should I call a technician for 40?

Call a technician if the error persists after resetting, or if you notice signs of electrical burning or unusual noises.

Sources & References

Data last verified: 2026-07-26

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