LG · Microwave Moderate

F-9 — Communication error between control boards

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.

Keep using it?
Yes, with caution
DIY difficulty
Moderate
Verified against
lg.com ↗

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.

Why it happens

Typical signs: F-9 on display · Display frozen or showing random segments · Buttons accepted but cycle won't start

  • Loose ribbon between display and main board Connection has worked loose, often after a recent service or shipping.
  • Damaged display board Failed micro on the sub-PCB.
  • Failed main control board Comm transceiver on the main PCB has failed.
  • Power supply noise Bad DC rail (5V) on the main board is causing comm dropouts.

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 display board Common replacement
    Amazon →
  • LG microwave main control board Common replacement
    Amazon →

More LG microwave codes

CH CH / Child Lock — Control locked DOOR DOOR / Door Error — Door switch / interlock fault F-1 F-1 — Main Control Board Fault F-2 F-2 — Keypad / touch panel shorted F-3 F-3 — Key short / specific button stuck F-4 F-4 — Humidity / steam sensor fault

All LG codes → All microwave codes →

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.