LG Microwave F4 Error — What It Means
The F4 error code on LG microwaves indicates a humidity sensor fault. The control system is not receiving a valid signal from the humidity sensor, which monitors moisture levels during cooking cycles. LG’s official guidance states this error requires service rather than user-level troubleshooting.
The fault usually lies in the humidity sensor itself, the wiring harness connecting it to the main control board, or the sensor input circuit on the control board. A valid signal cannot reach the controller, so the microwave halts operation and displays F4.
Common Causes
- Failed humidity sensor The sensor element has failed open, shorted, or drifted out of specification and no longer produces a valid signal.
- Loose or corroded connector The plug or terminals between the humidity sensor and control board have loosened, oxidized, or become contaminated, breaking the circuit.
- Damaged sensor wiring The harness running from the sensor to the main board is pinched, frayed, or broken, interrupting the signal path.
- Control board sensor-input fault The input circuit on the main control board that reads the humidity sensor has failed, even though the sensor and wiring are intact.
Step-by-Step Fix
- Disconnect power by unplugging the microwave from the wall outlet and wait one full minute to clear the control memory.
- Restore power and run a short cycle to confirm the F4 error returns, ruling out a transient glitch.
- Access the humidity sensor by removing the outer case panels (consult your model’s service manual for fastener locations and sensor placement, typically near the cavity exhaust).
- Inspect the sensor and harness for physical damage, burnt terminals, moisture contamination, or loose connector pins, and reseat the plug firmly.
- Test continuity and resistance from the sensor leads back to the control board connector using a multimeter, comparing readings to your model’s service data if available.
- Replace the humidity sensor if it tests open, shorted, or out of range, using the exact OEM part number for your microwave model.
- Replace the main control board if the sensor and wiring pass all tests but the F4 error persists, indicating a failed sensor input circuit on the board.
Parts Often Needed
| Part | Notes |
|---|---|
| LG microwave humidity sensor | Amazon | Order by your exact model number to match the connector and mounting style. |
| LG microwave main control board | Amazon | Required only if sensor and wiring test good but the error remains. |
When to Call a Pro
LG recommends professional service for F4 errors because diagnosis requires disassembly, multimeter testing of low-voltage circuits, and potential live-board work. If you are not comfortable removing cabinet panels, locating the humidity sensor, or interpreting resistance and continuity readings against service documentation, contact an authorized LG service technician or a qualified appliance repair shop. Always disconnect power before opening any panel, and be aware that improper handling of the control board or high-voltage components inside the microwave can cause injury or further damage.