LG · Microwave Serious

F-4 — Humidity / steam sensor fault

The humidity (steam) sensor used for Auto Cook and Sensor Reheat is out of range, open, or shorted. Sensor-based cooking is disabled until repaired.

Keep using it?
Limited use only
DIY difficulty
Moderate
Verified against
lg.com ↗

How to fix it

  1. Unplug for 60 seconds and retest.
  2. Open the cabinet and find the humidity sensor — usually a small block on the top duct near the cavity exhaust.
  3. Clean any greasy film off the sensor vent with isopropyl alcohol; do not soak the sensor itself.
  4. Verify the sensor plug is fully seated at both ends.
  5. If cleaning and reseating don't resolve, replace the humidity sensor.
  6. If the new sensor still throws F-4, replace the main control board.

Why it happens

Typical signs: F-4 on display · Sensor cook cycles end prematurely or refuse to start · Manual time-cook still works

  • Failed humidity / gas sensor The sensor element has aged or been fouled by smoke and grease.
  • Disconnected sensor harness Plug at the sensor or PCB has worked loose.
  • Blocked sensor vent Grease film over the sensor port prevents steam from reaching the element.
  • Main control board failure Bias circuit for the sensor on the PCB is bad.

Parts you may need

If diagnosis points to a failed part, these are the common replacements. As an Amazon Associate we earn from qualifying purchases.

  • LG microwave humidity / steam sensor Common replacement
    Amazon →
  • LG microwave main PCB Common replacement
    Amazon →

More LG microwave codes

DOOR DOOR / Door Error — Door switch / interlock fault F-1 F-1 — Main Control Board Fault F-2 F-2 — Thermistor / Temperature Out of Range F-3 F-3 — Key short / specific button stuck F-5 F-5 — Damper Switch Error F19 F19 — Inverter / High-Voltage Fault

All LG codes → All microwave codes →

Frequently asked questions

What does the F-4 code mean on a LG microwave?

The humidity (steam) sensor used for Auto Cook and Sensor Reheat is out of range, open, or shorted. Sensor-based cooking is disabled until repaired.

What causes the F-4 code on a LG microwave?

Failed humidity / gas sensor: The sensor element has aged or been fouled by smoke and grease. Disconnected sensor harness: Plug at the sensor or PCB has worked loose. Blocked sensor vent: Grease film over the sensor port prevents steam from reaching the element.

Is it safe to keep using a LG microwave with a F-4 error?

Limited use only. This is rated a serious-severity fault.

How do I fix F-4 on a LG microwave?

Unplug for 60 seconds and retest. Open the cabinet and find the humidity sensor — usually a small block on the top duct near the cavity exhaust. Clean any greasy film off the sensor vent with isopropyl alcohol; do not soak the sensor itself.

Always unplug an appliance before servicing it. This guide is for informational purposes and is not a substitute for a qualified technician. Verified against manufacturer documentation: lg.com.