F-9 Moderate Moderate DIY

LG Microwave F-9 — Communication error between control boards

LG Microwave · Error code F-9

What F-9 Means

The main control board and the display/sub board can't talk to each other. Microwave may show garbled display, freeze, or refuse to start.

Symptoms

  • F-9 on display
  • Display frozen or showing random segments
  • Buttons accepted but cycle won't start

Common Causes

1
Loose ribbon between display and main board

Connection has worked loose, often after a recent service or shipping.

2
Damaged display board

Failed micro on the sub-PCB.

3
Failed main control board

Comm transceiver on the main PCB has failed.

4
Power supply noise

Bad DC rail (5V) on the main board is causing comm dropouts.

How to Fix It

  1. Unplug for 60 seconds to reset.
  2. Open the control housing and unseat/reseat every ribbon and connector between the display board and the main board.
  3. Inspect ribbon for cracks or bent pins; replace if damaged.
  4. If reseating doesn't help, swap the display board first (cheaper); if F-9 persists, replace the main 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 display board View on Amazon→ Common replacement LG microwave main control board 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.