LG Microwave F4 Error Code: Humidity Sensor Error (What to Do)

What the code means

LG's microwave error code guide in its help library lists F4 as a humidity sensor error and marks it as needing service, alongside F1 (main board), F2 (thermistor), F3 (control panel), F5 (damper switch) and F11 (inverter communication). LG's general advice for any F code on its microwaves is that the symptom may be a temporary error, so unplug the cord, plug it back in and see whether the same code reappears; if it persists, call for an inspection. The humidity sensor matters because of how LG's sensor programs work: LG's owner's manuals explain that the sensor system works by detecting built-up steam from the food, and LG's help library says sensor cook and sensor reheat settings measure the humidity the food releases to decide when it is done. A sensor that can't be read, or reads out of range, leaves those programs with nothing to go on, so the control stops and shows F4. Service-level references describe the check the same way: the control watches the sensor warm up and settle to a reference voltage at the start of a sensor program, and flags F4 when it doesn't. The usual fixes are a new humidity sensor, a repaired sensor connector or harness, and, if a new sensor doesn't clear it, the control board. Note that on some older LG models the same sensor fault showed as E-4 rather than F4, and F5 on current LG models is a damper switch error, not a sensor code.

Likely causes

• A failed or degraded humidity sensor (the part LG's error guide names)
• A loose, corroded or damaged connector or wire between the sensor and the control board
• Grease and steam film built up on the sensor, which sits in the exhaust air path
• A control board that can't read the sensor correctly
• A temporary control glitch that clears after a power reset

Safe DIY checks

1. Unplug the microwave (or switch off its breaker) for two to three minutes, then plug it back in and try a sensor program again. LG says an error can be temporary.
2. Test a plain timed cook: put a cup of water in, set a time and power level, and press Start. If it heats normally, the microwave is still usable for timed cooking while you arrange a repair.
3. When you use sensor programs, follow LG's container rules: a microwave-safe dish, loosely covered with a vented lid or vented plastic wrap, never a tight-sealed cover, and the outside of the dish and inside of the oven dry, since beads of moisture can mislead the sensor.
4. Keep the interior clean, especially after greasy foods like bacon. LG suggests wiping spatters right away and boiling a cup of water for 2 to 3 minutes to loosen stubborn soil.
5. On an over-the-range model, make sure the vent grille and grease filters aren't clogged so steam can move through the unit.
6. Don't open the cabinet to reach the sensor. Microwaves contain a high-voltage capacitor that can hold a dangerous charge even when unplugged.

Parts that commonly fix it

Always match the part number to your exact model.

Related codes

• F3 LG Microwave F3
• LOC LG Microwave LOC
• F3 GE Microwave F3
• PF GE Microwave PF
• C-10 Samsung Microwave C-10

All LG codes →

Sources

1. LG Help Library: Microwave - Error Code Cause & Solutions (LG's error code chooser listing F1 to F19 codes; F4 humidity sensor error, service required)
2. LG USA Support: LG Microwave - Troubleshooting an F3 Error (F codes, temporary errors, unplug and replug, call for inspection if it persists)
3. LG MSER2090/MSER1590 Owner's Manual (sensor system works by detecting built-up steam; loose vented covers; moisture beads can mislead the sensor; cleaning)
4. LG Canada Help Library: Sensor cooking [countertop] (sensor programs measure the humidity food releases)
5. Fault Code Check: LG F4 microwave fault code (LG publishes F4 as a humidity sensor error requiring service; timed cooking usually still works)
6. LG Magazine Russia: LG microwave error codes (older E-4 code = humidity sensor problem)