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Error code reference

LG Hvac Error Code E121

Indoor temperature sensor error (short or open circuit).

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Indoor temperature sensor error (short or open circuit).

First step from the service manual

Check if temperature sensor connector (CN41) 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 E121 means on a LG hvac

The E121 error code on an LG HVAC unit indicates a fault with the indoor temperature sensor, specifically that the sensor circuit is either shorted or open. The indoor temperature sensor is a thermistor, a resistive component whose electrical resistance changes in proportion to the surrounding air temperature. The PCB (printed circuit board) continuously monitors the resistance signal from this thermistor to determine indoor ambient temperature and regulate system operation accordingly. When the resistance value falls outside the expected range, either dropping too low (short circuit) or going completely absent (open circuit), the system flags the E121 fault and interrupts normal operation.

The sensor connects to the control board via a dedicated connector designated CN41. A short circuit condition typically means the sensor's resistance has dropped to near zero, often caused by physical damage to the sensor body or wiring. An open circuit means the electrical path is completely broken, which can result from a disconnected or corroded connector, a broken wire in the harness, or a failed thermistor. Because the board receives no usable temperature data, it cannot safely manage heating or cooling cycles and will shut down to prevent damage.

Diagnosis begins at connector CN41 on the PCB. A displaced or loosely seated connector is the most common and easiest-to-resolve cause of this fault. If reconnecting the connector does not clear the error, the sensor resistance should be measured with a multimeter and compared against the temperature-resistance specification curve for the unit. Wiring continuity between the sensor and the board should also be confirmed before concluding the sensor itself has failed.

Source: manufacturer service documentation.

Common causes of E121

  1. 01

    Disconnected sensor connector at CN41

    Wiring / connection

    The connector that attaches the indoor temperature sensor to the PCB at position CN41 may have worked loose over time due to vibration. Visually inspect and firmly reseat the connector to resolve this.

  2. 02

    Failed indoor temperature sensor (thermistor)

    Part replacement

    The thermistor itself may have failed, resulting in either a short or open circuit reading. Testing the sensor's resistance with a multimeter and comparing it to the rated specification will confirm this fault.

  3. 03

    Damaged or broken sensor wiring harness

    Part replacement

    The wiring between the sensor and the PCB may have a break or pinch point causing an open or intermittent circuit. Inspect the full length of the wire harness for visible damage, kinks, or pinching.

  4. 04

    Corroded or oxidized connector pins

    Wiring / connection

    Moisture exposure can cause corrosion at the CN41 connector pins, increasing resistance or breaking the circuit entirely. Inspect the connector pins for discoloration, oxidation, or buildup and clean or replace as needed.

  5. 05

    Faulty indoor PCB

    Part replacement

    If the sensor and wiring test within specification but the error persists, the PCB itself may have a fault on the sensor input circuit. This is the least common cause and should only be considered after ruling out the sensor and wiring.

See the test procedure for each cause

Frequently asked questions about E121

What does E121 mean on a LG hvac?
E121 indicates an indoor temperature sensor error caused by a short or open circuit in the sensor circuit. The system uses this sensor to monitor indoor ambient temperature and control operation. When the sensor signal is out of range or absent, the unit logs E121 and stops normal operation to prevent running without accurate temperature data.
How do I fix the E121 error on my LG HVAC unit?
Start by checking whether the sensor connector at CN41 on the indoor PCB has come loose, and firmly reseat it if so. If the connector is secure, use a multimeter to test the resistance of the temperature sensor to determine if it has shorted or failed open. A damaged wiring harness or corroded connector pins can also cause this error and should be inspected. If the sensor or wiring is faulty, replacing the defective component is required.
Is it safe to run my LG HVAC with an E121 error?
The unit will typically shut down or refuse to operate normally when E121 is active, precisely because it cannot regulate temperature without a working sensor. Attempting to force operation without resolving the fault is not recommended, as the system lacks the data needed to control heating or cooling cycles safely. Address the underlying sensor or wiring issue before returning the unit to service.
Will resetting my LG HVAC clear the E121 error?
A power reset may temporarily clear the error display, but the fault will return immediately if the underlying sensor or wiring problem has not been corrected. A reset is only useful as a final step to confirm the repair after the sensor, connector, or wiring has been inspected and any issues resolved. Repeated resets without repair will not fix the problem.
How much does it cost to repair an E121 error on an LG HVAC?
If the fix is simply reseating the CN41 connector, there may be no parts cost at all, though a service call will typically run between $75 and $150 for labor. If the indoor temperature sensor thermistor needs replacement, parts generally range from $15 to $60 depending on the specific model. A PCB replacement, if required, is significantly more expensive and can range from $150 to $400 or more for parts alone.

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