mypros+
Error code reference

LG Hvac Error Code 3

10/13 SEER Flex Multi

LG codesLG hvac codes

10/13 SEER Flex Multi: Different mode operation (IDUs must be all heat or all cool). Inverter: Communication error between IDU & hard-wired controller. PTAC: PCB sees outdoor air thermistor as open or shorted.

First step from the service manual

Inverter: Ensure only LG factory wire is used & properly plugged from IDU PCB to controller. PTAC: Must be 5,600Ω at 38°C/100°F & 20,700Ω at 10°C/50°F.

The complete diagnostic procedure includes additional test steps, resistance specifications, wiring diagram references, and component test points.

MyPros+ Max

Get the full diagnostic procedure for 3.

  • Step-by-step tests with expected resistance, voltage, and continuity values
  • Wiring diagram references and connector pinouts from the OEM service manual
  • Verified part numbers when replacement is required
Get the full procedure

$29/month · Cancel anytime · Or $9.95 3-day pass

What 3 means on a LG hvac

Error code 3 on LG HVAC systems covers three distinct fault conditions depending on the system type. On 10/13 SEER Flex Multi systems, this code indicates a mode conflict where indoor units (IDUs) are not all operating in the same mode. Every IDU connected to the system must be set to either all heat or all cool simultaneously. Mixed mode operation is not supported and will trigger this fault to protect the refrigerant circuit from conflicting demands.

On LG Inverter systems, error code 3 signals a communication failure between the indoor unit PCB and the hard-wired controller. This type of fault is typically caused by incorrect wiring, a damaged communication cable, or the use of non-LG wiring between the IDU and the controller. The system requires LG factory wire to maintain signal integrity, and any substitution or loose connection can interrupt the communication protocol.

On PTAC units, error code 3 means the main PCB has detected that the outdoor air thermistor is reading as either open or shorted. At 38 degrees C (100 degrees F), the thermistor should measure approximately 5,600 ohms. At 10 degrees C (50 degrees F), the correct resistance is approximately 20,700 ohms. Readings outside these values indicate a failed thermistor or a wiring fault between the sensor and the PCB.

Source: manufacturer service documentation.

Common causes of 3

  1. 01

    Outdoor air thermistor failure (PTAC)

    Part replacement

    The outdoor air thermistor has failed open or shorted, causing resistance readings outside the specified range of 5,600 ohms at 38 degrees C or 20,700 ohms at 10 degrees C. Test the thermistor with a multimeter and compare to the reference values to confirm failure.

  2. 02

    Non-LG or incorrect communication wire (Inverter)

    Wiring / connection

    Using aftermarket or incorrect wire between the IDU PCB and the hard-wired controller can disrupt the communication signal and trigger this fault. Only LG factory wire should be used for this connection.

  3. 03

    Loose or improperly seated communication connector (Inverter)

    Wiring / connection

    Even with the correct wire, a connection that is not fully seated at either the IDU PCB or the controller terminal can cause intermittent or persistent communication errors. Inspect and firmly reseat all connectors.

  4. 04

    Mixed heating and cooling mode on Flex Multi IDUs

    Wiring / connection

    On 10/13 SEER Flex Multi systems, one or more indoor units is set to a different operating mode than the others. All IDUs must be set to the same mode, either all heat or all cool, for the system to operate without this fault.

  5. 05

    Thermistor wiring damage or short to ground (PTAC)

    Wiring / connection

    Damaged insulation or pinched wiring between the outdoor thermistor and the PCB can create a short or open circuit that mimics a sensor failure. Inspect the wiring harness for visible damage and verify continuity.

See the test procedure for each cause

Frequently asked questions about 3

What does 3 mean on a LG hvac?
Error code 3 on LG HVAC systems indicates one of three faults depending on the unit type. On Flex Multi systems it means indoor units are operating in conflicting modes. On Inverter systems it signals a communication error between the indoor unit PCB and the hard-wired controller. On PTAC units it means the outdoor air thermistor is reading as open or shorted.
How do I fix error code 3 on my LG HVAC system?
The fix depends on your system type. For Flex Multi systems, ensure all indoor units are set to the same mode, either all heat or all cool. For Inverter systems, confirm that only LG factory wire is being used and that it is fully and correctly plugged in at both the IDU PCB and the controller. For PTAC units, test the outdoor air thermistor with a multimeter and compare readings to the specified values of 5,600 ohms at 38 degrees C and 20,700 ohms at 10 degrees C.
Is error code 3 on a LG HVAC dangerous?
Error code 3 is not an immediately dangerous fault, but it does prevent the system from operating correctly. On multi-zone systems, mixed mode operation can stress the refrigerant circuit over time. A failed thermistor on a PTAC unit means the system cannot accurately monitor outdoor air temperature, which can affect performance and efficiency. It is best to diagnose and resolve the fault promptly.
How much does it cost to repair LG HVAC error code 3?
Repair costs vary based on the cause. If the fix is simply correcting indoor unit modes or reseating a loose connector, there may be little to no parts cost, though a service call fee typically ranges from 75 to 150 dollars. Replacing an outdoor air thermistor on a PTAC unit generally costs between 20 and 80 dollars for the part, plus labor. Using incorrect communication wire on an Inverter system may require purchasing LG factory replacement wire, which is typically a modest cost.
Will resetting my LG HVAC clear error code 3?
A reset may temporarily clear the code, but it will return if the underlying issue is not resolved. For Flex Multi mode conflicts, the error will reappear as soon as the system detects mismatched IDU modes again. For Inverter communication faults and PTAC thermistor problems, the error will come back on the next startup or operational cycle. A proper diagnosis and repair of the root cause is required to permanently clear the fault.

Related LG hvac codes

All LG hvac codes
View all 75 LG hvac codes

78,000+ service manuals · 160,000+ error codes · 55+ brands

Get Started

$29/month · Cancel anytime · Or $9.95 3-day pass