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Error code reference

LG Hvac Error Code 4

LA09xHV/LA12xHV/LS902HPV/LS122HPV

LG codesLG hvac codes

LA09xHV/LA12xHV/LS902HPV/LS122HPV: ODU PCB heat sink thermistor open, shorted or overheated to 95°C. Other units: IDU condensate pump float switch risen/opened.

First step from the service manual

Heat sink thermistor must be 5,600Ω at 38°C/100°F & 20,700Ω at 10°C/50°F. For float switch: check drain pan is empty & pump working; if no pump, ensure blue jumper plug inserted in IDU PCB plug CN_FLOAT.

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

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What 4 means on a LG hvac

Error code 4 on LG HVAC systems covers two distinct faults depending on the unit model. On LA09xHV, LA12xHV, LS902HPV, and LS122HPV outdoor units, this code indicates a fault with the outdoor unit (ODU) PCB heat sink thermistor. The thermistor monitors the temperature of the power module heat sink and the error triggers when the sensor reads an open circuit, a short circuit, or an actual overtemperature condition where the heat sink reaches 95°C. The heat sink thermistor is a negative temperature coefficient (NTC) type resistor, and correct resistance values are 5,600 ohms at 38°C (100°F) and 20,700 ohms at 10°C (50°F). Readings outside this range indicate a failed thermistor, while a reading within spec points to genuine overheating of the power module.

On all other LG unit models, error code 4 points to the indoor unit (IDU) condensate pump float switch circuit. When the condensate drain pan fills beyond its safe level, the float switch rises and opens the circuit, signaling the control board to shut down to prevent water overflow damage. This protective function is intentional and normal, but a persistent code indicates either the drain pan is not emptying, the condensate pump has failed, or the float switch circuit itself has an issue.

A critical configuration note applies to indoor units that do not use a condensate pump: the IDU PCB plug labeled CN_FLOAT must have a blue jumper plug installed. Without this jumper, the control board interprets the open circuit as a raised float switch and locks out on error code 4 even when no drain issue exists. Always confirm jumper installation before replacing any components on non-pump units.

Source: manufacturer service documentation.

Common causes of 4

  1. 01

    Condensate drain pan full or blocked drain

    Wiring / connection

    A clogged drain line causes water to accumulate in the pan, raising the float switch and triggering the lockout. Inspect the drain pan for standing water and clear any blockage in the drain line or trap.

  2. 02

    Missing CN_FLOAT jumper plug on non-pump units

    Wiring / connection

    Indoor units without a condensate pump require a blue jumper plug installed in the PCB's CN_FLOAT connector. If this jumper is absent, the board reads the open circuit as a fault and displays error code 4 immediately.

  3. 03

    Failed condensate pump

    Part replacement

    If a condensate pump is installed but has stopped operating due to motor failure or a clog in the pump itself, water will back up in the drain pan and raise the float switch. Test the pump for power and mechanical operation.

  4. 04

    Heat sink thermistor failure (LA09xHV, LA12xHV, LS902HPV, LS122HPV only)

    Part replacement

    On the listed outdoor unit models, the NTC thermistor mounted on the power module heat sink can fail open or short. Measure resistance across the thermistor: values other than approximately 5,600 ohms at 38°C or 20,700 ohms at 10°C confirm a failed sensor requiring replacement.

  5. 05

    Outdoor unit power module overheating (LA09xHV, LA12xHV, LS902HPV, LS122HPV only)

    Wiring / connection

    If the thermistor resistance checks within specification, the heat sink itself may be reaching 95°C due to inadequate airflow, a dirty heat exchanger, or a failing fan motor. Inspect the outdoor unit for blocked airflow and debris on the coil.

See the test procedure for each cause

Frequently asked questions about 4

What does 4 mean on a LG hvac?
Error code 4 on LG HVAC systems means one of two things depending on the model. On LA09xHV, LA12xHV, LS902HPV, and LS122HPV units, it indicates the outdoor unit PCB heat sink thermistor has failed or the heat sink has reached 95°C. On all other LG models, it means the indoor unit condensate float switch has opened, which is typically caused by a full drain pan, a failed condensate pump, or a missing jumper plug on the indoor PCB.
Can I fix error code 4 on my LG HVAC myself?
Some causes of error code 4 are accessible to a knowledgeable DIYer. Clearing a blocked drain line, checking the condensate pump for operation, and verifying the blue jumper plug is seated in the CN_FLOAT connector on the indoor PCB are all reasonable first steps. Replacing the outdoor unit heat sink thermistor or diagnosing power module overheating on the affected outdoor models is more involved and may require a qualified technician.
Is error code 4 on a LG HVAC dangerous?
On indoor units, error code 4 is a protective shutdown to prevent water from overflowing the drain pan and causing water damage or mold growth. On outdoor units, the code protects the power electronics from heat damage. Neither condition is immediately dangerous to occupants, but the system should not be forced to run continuously while the fault is present.
How much does it cost to repair error code 4 on a LG HVAC?
Costs vary widely depending on the root cause. Clearing a drain line or installing a missing jumper plug may cost little to nothing if done by the homeowner. Condensate pump replacement typically runs in the range of $100 to $300 including labor. Heat sink thermistor replacement on outdoor units generally falls between $150 and $400 depending on parts availability and technician rates.
Will resetting the unit clear error code 4 on a LG HVAC?
A power reset may temporarily clear the display, but the error code will return if the underlying fault is not corrected. For float switch faults, the drain pan must be emptied and the drainage system restored to proper function before the system will operate normally. For thermistor or overheating faults on the outdoor unit models, the failed component must be repaired or replaced to achieve a lasting fix.

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