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Error code reference

LG Microwave Error Code F-4

Humidity sensor self-heating time exceeds 64 seconds or zero balancing voltage does not rise to 2.5V during…

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Humidity sensor self-heating time exceeds 64 seconds or zero balancing voltage does not rise to 2.5V during sensor cook operation.

First step from the service manual

Replace humidity sensor. If F-4 repeats after sensor replacement, replace the PC Board.

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

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  • Step-by-step tests with expected resistance, voltage, and continuity values
  • Wiring diagram references and connector pinouts from the OEM service manual
  • Verified part numbers when replacement is required
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What F-4 means on a LG microwave

The F-4 error code on an LG microwave indicates a fault within the humidity sensor circuit during a sensor cook operation. Specifically, the error triggers when one of two conditions occurs: either the humidity sensor's self-heating cycle takes longer than 64 seconds to complete, or the zero balancing voltage in the sensor circuit fails to reach the required 2.5V threshold. The humidity sensor works by detecting moisture levels in the steam released from food during cooking, allowing the microwave to automatically adjust cooking time and power. When the sensor cannot properly calibrate or heat itself within the specified window, the control board flags the fault and halts the operation.

The most likely cause of an F-4 error is a failed or degraded humidity sensor. Over time, the sensor element can become contaminated by food residue, grease, or steam deposits, which interfere with its electrical characteristics and prevent it from reaching proper operating voltage. A faulty sensor may also have an open or high-resistance internal circuit that slows or prevents self-heating. Physical damage from moisture ingress or heat exposure can also cause the sensor to behave erratically.

If replacing the humidity sensor does not resolve the F-4 error, the fault lies with the PC board. The board is responsible for supplying the correct voltage to the sensor circuit and monitoring the self-heating and balancing voltage parameters. A damaged control board may fail to deliver adequate drive voltage to the sensor, misread the balancing voltage, or have a faulty input circuit. In this case, the PC board itself must be replaced to restore proper sensor cook functionality.

Source: manufacturer service documentation.

Common causes of F-4

  1. 01

    Failed or degraded humidity sensor

    Part replacement

    The humidity sensor is the primary suspect for F-4. The sensor element may have failed internally, become contaminated with grease or food residue, or developed an open circuit that prevents it from self-heating within the 64-second limit.

  2. 02

    Sensor contamination from cooking residue

    Wiring / connection

    Heavy buildup of grease or steam deposits on the sensor surface can alter its electrical properties and prevent the zero balancing voltage from reaching the 2.5V threshold required by the control board.

  3. 03

    Poor or corroded sensor wiring connection

    Wiring / connection

    A loose, corroded, or damaged connector between the humidity sensor and the PC board can disrupt the voltage signal and cause intermittent or persistent F-4 errors. Inspect the connector pins and harness for corrosion or damage.

  4. 04

    Faulty PC board

    Part replacement

    If the humidity sensor has been replaced and the F-4 code returns, the PC board is failing to properly supply or monitor the sensor circuit voltage. The board must be replaced in this case.

See the test procedure for each cause

Frequently asked questions about F-4

What does F-4 mean on a LG microwave?
The F-4 code indicates a problem with the humidity sensor during a sensor cook operation. It means either the sensor's self-heating cycle exceeded 64 seconds or the zero balancing voltage in the sensor circuit did not rise to the required 2.5V. The microwave cannot complete sensor-based cooking until the fault is resolved. The humidity sensor or the PC board is typically responsible.
Can I fix the F-4 error myself on my LG microwave?
Replacing the humidity sensor is the recommended first step and is a repair that a technically comfortable DIYer can attempt after unplugging the unit. If the error persists after a new sensor is installed, the PC board requires replacement, which is a more involved repair. Always disconnect power before opening the microwave cabinet, and be aware that the capacitor inside can hold a dangerous charge even after unplugging.
Will resetting my LG microwave clear the F-4 error?
Unplugging the microwave and restoring power may temporarily clear the F-4 display, but the code will return as soon as a sensor cook cycle is initiated if the underlying hardware fault is not corrected. A reset does not fix a failed humidity sensor or a faulty PC board. The error will persist until the defective component is replaced.
Is it safe to use my LG microwave while it is showing an F-4 error?
The microwave's sensor cook functions will not operate reliably with an F-4 fault present. Manual time cooking may still function, but sensor-based cooking modes should be avoided until the fault is repaired. There is no immediate safety hazard from the error itself, but operating the unit with a known component fault is not recommended.
How much does it cost to repair the F-4 error on an LG microwave?
If only the humidity sensor requires replacement, parts and labor typically range from around $75 to $175 depending on the model and service rates in your area. If the PC board also needs replacement, total repair costs can range from $150 to $350 or more. For older units, it is worth comparing repair costs against the price of a replacement microwave before proceeding.

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