F-4 Serious Moderate DIY

LG Microwave F-4 — Humidity / steam sensor fault

LG Microwave · Error code F-4

What F-4 Means

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.

Symptoms

  • F-4 on display
  • Sensor cook cycles end prematurely or refuse to start
  • Manual time-cook still works

Common Causes

1
Failed humidity / gas sensor

The sensor element has aged or been fouled by smoke and grease.

2
Disconnected sensor harness

Plug at the sensor or PCB has worked loose.

3
Blocked sensor vent

Grease film over the sensor port prevents steam from reaching the element.

4
Main control board failure

Bias circuit for the sensor on the PCB is bad.

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.

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.

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

More LG Microwave Codes

CH CH / Child Lock — Control locked DOOR DOOR / Door Error — Door switch / interlock fault EE EE — EEPROM / control memory error F-1 F-1 — Oven sensor open / thermistor fault F-2 F-2 — Keypad / touch panel shorted F-3 F-3 — Key short / specific button stuck

All LG codes → All microwave codes →

Verified against manufacturer documentation: lg.com

Always unplug an appliance before servicing it. This guide is for informational purposes and is not a substitute for a qualified technician.