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Error code reference

LG Hvac Error Code CO

PCB sees IDU air thermistor as open or shorted.

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PCB sees IDU air thermistor as open or shorted.

First step from the service manual

Ohm thermistor in water, measure the water temperature; must be 5,600Ω at 38°C/100°F & 20,700Ω at 10°C/50°F. With perfect thermistor & plug connection, change PCB.

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

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What CO means on a LG hvac

The CO error code on an LG indoor unit (IDU) indicates that the main PCB has detected an abnormal signal from the indoor air thermistor. Specifically, the PCB is reading the thermistor circuit as either open (infinite resistance, indicating a broken wire or failed sensor element) or shorted (near-zero resistance, indicating an internal sensor failure or wiring fault). The air thermistor is a negative temperature coefficient (NTC) resistor responsible for monitoring return air temperature, and its readings are used by the PCB to regulate compressor operation, fan speed, and temperature control logic.

The correct resistance profile for this thermistor is 20,700 ohms at 10 degrees Celsius (50 degrees Fahrenheit) and 5,600 ohms at 38 degrees Celsius (100 degrees Fahrenheit). To validate the sensor, it should be submerged in water at a known temperature and measured with a multimeter. Any reading that falls significantly outside these reference points at their corresponding temperatures confirms sensor failure. A broken or corroded connector at the thermistor plug can also produce an open-circuit reading even when the thermistor element itself is intact.

If the thermistor measures within specification and the plug connection is clean and fully seated, the fault lies with the PCB itself, which is misinterpreting a valid signal. In that case, the PCB must be replaced. This logical sequence, testing the thermistor first and then attributing the fault to the PCB only after the thermistor is confirmed good, is the correct diagnostic path for this error code.

Source: manufacturer service documentation.

Common causes of CO

  1. 01

    Failed air thermistor

    Part replacement

    The NTC thermistor element itself has failed, producing a resistance value that is either extremely high (open) or near zero (shorted). Confirm by measuring resistance in water at a known temperature and comparing to 20,700 ohms at 10°C or 5,600 ohms at 38°C.

  2. 02

    Loose or corroded thermistor connector

    Wiring / connection

    The plug connecting the thermistor to the PCB may be partially unseated, corroded, or have a broken pin, causing the PCB to read an open circuit. Inspect and reseat the connector and check for visible corrosion or bent terminals.

  3. 03

    Damaged thermistor wiring

    Part replacement

    The wire harness between the thermistor and the PCB may have a break or short caused by pinching, rodent damage, or rubbing against a sharp edge. Visually trace the wire run and check for any obvious damage.

  4. 04

    Faulty PCB

    Part replacement

    If the thermistor measures correctly at both reference temperatures and the connector is fully intact, the PCB is misreading the valid signal. Board replacement is required in this scenario.

See the test procedure for each cause

Frequently asked questions about CO

What does CO mean on a LG hvac?
The CO code means the indoor unit's main PCB has detected that the air thermistor circuit is open or shorted. The thermistor is the sensor that monitors return air temperature inside the unit. The code points to either a failed thermistor, a wiring or connector problem, or in some cases a faulty PCB.
Can I fix the CO error code myself?
A knowledgeable DIYer can perform the thermistor resistance test by submerging the sensor in water at a known temperature and measuring resistance with a multimeter. The target values are 20,700 ohms at 10°C (50°F) and 5,600 ohms at 38°C (100°F). Connector inspection and reseating is also a straightforward check. However, if the PCB needs replacement, that repair is best handled by a qualified technician.
Is it safe to run my LG HVAC with the CO error code active?
The system may shut down or refuse to operate normally when the CO code is active because the PCB cannot reliably read air temperature. Running the unit in this condition risks improper temperature control and potential compressor stress. It is best to diagnose and resolve the fault before continuing regular operation.
How much does it cost to repair the CO error on an LG HVAC?
If the thermistor is the cause, sensor replacement typically costs between $50 and $150 including parts and labor. If the PCB is at fault, repair costs generally range from $200 to $600 or more depending on the unit model and labor rates in your area. Getting a diagnosis first will help clarify which component needs replacement before committing to a repair cost.
Will resetting the unit clear the CO error code?
A power reset may temporarily clear the code, but it will return as soon as the PCB polls the thermistor circuit again and detects the same open or shorted condition. The underlying hardware fault must be corrected before the error will stay cleared. Resetting without repairing is not a solution.

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