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Error code reference

LG Oven Error Code F-4

Lower oven thermistor remains open for over 1 minute after cook starts.

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Lower oven thermistor remains open for over 1 minute after cook starts.

First step from the service manual

Check the thermistor using test mode 4.Diagnosis; verify sensor value is 70°F–90°F at room temperature. If not, power off and measure thermistor resistance at CN05 — normal is approximately 1.09 kΩ.

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

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What F-4 means on a LG oven

The F-4 error code on an LG oven indicates that the lower oven thermistor has remained in an open circuit condition for more than one minute after a cooking cycle begins. The thermistor is a temperature-sensing resistor that continuously reports oven cavity temperature to the control board. When the circuit reads as open, the control board receives no valid temperature signal and cannot regulate heating safely, so it shuts down the cook cycle and displays F-4 to alert the user.

At room temperature, a properly functioning lower oven thermistor should measure approximately 1.09 kΩ of resistance at connector CN05. This resistance value drops predictably as oven temperature rises, allowing the control board to track temperature in real time. An open circuit means resistance is effectively infinite, which the board interprets as a broken or disconnected sensor. This can result from a failed thermistor, damaged wiring, a loose connector, or corrosion at the CN05 connector terminals.

Diagnosis begins in test mode 4, where the sensor value should read between 70°F and 90°F at ambient room temperature. If the displayed value is outside that range or absent, the next step is to power down the oven and measure resistance directly at CN05 with a multimeter. A reading near 1.09 kΩ suggests the thermistor itself is intact and the wiring harness or control board connection may be the fault point. A reading far outside that value, or an open-loop reading, confirms thermistor failure.

Source: manufacturer service documentation.

Common causes of F-4

  1. 01

    Failed lower oven thermistor

    Part replacement

    The thermistor itself has broken internally, causing an open circuit. Resistance measured at CN05 will read far outside the expected 1.09 kΩ value or show no continuity at all.

  2. 02

    Loose or disconnected wiring at CN05

    Wiring / connection

    The connector at CN05 may have worked loose or been pulled free, interrupting the sensor circuit. Visually inspect the connector and reseat it firmly before replacing any parts.

  3. 03

    Damaged wiring harness

    Part replacement

    Heat exposure or physical contact with the oven cavity can melt, pinch, or break the wires running from the thermistor to the control board. Inspect the harness for visible burns, fraying, or breaks.

  4. 04

    Corroded or oxidized connector terminals at CN05

    Wiring / connection

    Moisture or grease contamination over time can corrode the terminals inside the CN05 connector, creating high resistance or an open connection. Clean terminals carefully and retest resistance.

  5. 05

    Faulty control board

    Part replacement

    If the thermistor measures correctly at approximately 1.09 kΩ at room temperature and wiring checks out, the control board may not be reading the CN05 input properly. Board replacement would be required in this case.

See the test procedure for each cause

Frequently asked questions about F-4

What does F-4 mean on a LG oven?
F-4 means the lower oven thermistor has been detected as an open circuit for more than one minute after a cooking cycle starts. The thermistor is the temperature sensor that tells the control board how hot the oven cavity is. Without a valid reading, the oven cannot regulate heat safely and shuts down. The fault points to the thermistor, its wiring, or the CN05 connector.
Can I fix an F-4 error on my LG oven myself?
Technicians and experienced DIYers can diagnose this fault using test mode 4 to check the sensor reading, then measuring resistance at connector CN05 with a multimeter. The expected room-temperature resistance is approximately 1.09 kΩ. If the thermistor is confirmed failed, replacing it is a relatively straightforward repair, but all work should be done with the oven fully powered off and unplugged.
Is it safe to use my LG oven while the F-4 error is displayed?
No. The oven disables the cooking function precisely because it cannot safely monitor cavity temperature without the thermistor signal. Attempting to bypass or ignore this fault could result in uncontrolled overheating. The oven should not be used until the sensor circuit is repaired and tested.
Will resetting my LG oven clear the F-4 error?
A power reset may temporarily clear the fault code, but F-4 will return as soon as the next cook cycle starts if the underlying problem is not fixed. The control board checks the thermistor signal within the first minute of operation, so any open circuit condition will trigger the code again quickly. A reset is only useful as a diagnostic step, not a repair.
How much does it cost to repair an F-4 error on a LG oven?
If only the thermistor needs replacement, parts and labor typically range from around $100 to $250 depending on your location and service provider. If the wiring harness is damaged, costs can rise into the $150 to $350 range. A control board replacement, if required, is generally the most expensive outcome, often falling between $200 and $450 or more including labor.

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