LG HVAC Error Code E139

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.

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.

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.

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.

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.