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Error code reference

LG Hvac Error Code E139

CO2 sensor error (short or open circuit); CO2 input pulse maintains High or Low status for longer than 2…

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CO2 sensor error (short or open circuit); CO2 input pulse maintains High or Low status for longer than 2 minutes.

First step from the service manual

Check if CO2 sensor connector (CN43) is dislocated from PCB; if so, reconnect it.

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

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What E139 means on a LG hvac

The E139 error code on LG HVAC systems indicates a fault with the CO2 sensor circuit. Specifically, the control board has detected that the CO2 input pulse signal has remained in a continuously High or Low state for more than 2 minutes, which signals either a short circuit or an open circuit condition within the sensor or its wiring. The CO2 sensor is used to monitor indoor air quality and carbon dioxide concentration, allowing the system to adjust ventilation accordingly. When the sensor signal is stuck and fails to cycle normally, the PCB interprets this as a sensor failure and triggers the E139 fault.

The most immediate cause to investigate is a dislodged or poorly seated connector at CN43, which is the designated CO2 sensor connector on the PCB. A loose or disconnected CN43 connector will break the signal path entirely, causing the input pulse to hold at a fixed state. Beyond the connector itself, damaged wiring between the sensor and the board, a failed CO2 sensor, or a fault on the PCB input circuit can all produce the same stuck-signal condition. Environmental contamination or moisture ingress near the sensor or connector can accelerate contact corrosion and contribute to intermittent or permanent open circuit faults.

Diagnosis should begin at the CN43 connector before moving to component-level testing. Inspect the connector for proper seating, bent pins, corrosion, or damaged locking tabs. If the connector is intact, trace the wiring harness for breaks or chafing. If wiring and connections are confirmed good, the CO2 sensor itself should be tested or replaced. A fault isolated to the PCB input circuit would require board-level evaluation or board replacement.

Source: manufacturer service documentation.

Common causes of E139

  1. 01

    Disconnected CO2 sensor connector (CN43)

    Wiring / connection

    The CN43 connector on the PCB may have worked loose over time due to vibration or handling. A dislodged connector immediately breaks the signal path, causing the stuck High or Low input condition that triggers E139.

  2. 02

    Damaged or broken sensor wiring

    Wiring / connection

    The wiring harness between the CO2 sensor and the PCB may have chafed, pinched, or corroded contacts, creating an open or short circuit. Inspect the full length of the harness for visible damage or areas of wear.

  3. 03

    Failed CO2 sensor

    Part replacement

    The CO2 sensor itself may have failed internally, causing it to output a constant High or Low signal rather than a cycling pulse. A failed sensor will need to be replaced to restore normal operation.

  4. 04

    Corroded or contaminated connector contacts

    Wiring / connection

    Moisture or airborne contaminants can corrode the pins inside the CN43 connector, increasing resistance or breaking continuity. Inspect the connector pins closely and clean or replace as needed.

  5. 05

    PCB input circuit fault

    Part replacement

    If the connector, wiring, and sensor all test correctly, the fault may lie in the PCB input circuit that reads the CO2 signal. This would require evaluation and likely replacement of the control board.

See the test procedure for each cause

Frequently asked questions about E139

What does E139 mean on a LG hvac?
E139 indicates a CO2 sensor error caused by a short or open circuit in the sensor circuit. The control board detected that the CO2 input pulse signal remained stuck in a High or Low state for more than 2 minutes without cycling. This prevents the system from accurately monitoring indoor air quality. The fault can originate at the sensor connector, wiring, the sensor itself, or the PCB.
How do I fix the E139 error on my LG HVAC system?
Start by checking the CO2 sensor connector labeled CN43 on the PCB and firmly reseat it if it is loose or disconnected. Next, inspect the wiring harness from the sensor to the board for any visible damage, pinching, or corrosion. If the connections are secure and the wiring is intact, the CO2 sensor itself may need to be tested and replaced. If replacing the sensor does not clear the fault, the PCB input circuit should be evaluated by a qualified technician.
Is the E139 error on a LG HVAC a safety hazard?
The E139 error itself is a sensor circuit fault, not a direct indication of dangerous CO2 levels in your space. However, with the CO2 sensor inoperable, the system cannot properly regulate ventilation based on air quality. It is best to resolve the fault promptly so the ventilation function is restored to normal operation.
How much does it cost to repair an E139 error on a LG HVAC unit?
If the fix is as simple as reseating the CN43 connector, there may be no parts cost at all, just a service call fee which typically ranges from $75 to $150 depending on your area. A replacement CO2 sensor generally costs between $50 and $200 for the part, plus labor. If the PCB requires replacement, total costs could range from $300 to $700 or more depending on the specific unit and labor rates.
Will resetting the LG HVAC unit clear the E139 error code?
A power reset may temporarily clear the E139 code from the display, but the error will return if the underlying sensor circuit fault has not been corrected. The code is triggered by a persistent signal condition lasting more than 2 minutes, so any unresolved wiring, connector, or sensor issue will cause it to reappear. A proper repair of the root cause is required for the code to stay cleared.

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