Microwave will not start; display shows F1 shortly after pressing START or when plugging in the unit.

Safety first: Unplug the microwave from the wall outlet before any inspection. Do not remove the outer cabinet or touch internal high-voltage components; capacitors can retain a dangerous charge even when unplugged.

Step-by-step checks (user level):

• Power reset: Unplug the microwave for 5–10 minutes to allow the control board to fully discharge. Plug it back in and see if F1 reappears.
• Check for stuck keys: Gently press each button on the keypad. If any button feels stuck or does not click, wipe the panel with a soft, slightly damp cloth and dry thoroughly. Avoid spraying cleaner directly on the keypad.
• Inspect keypad ribbon (if accessible from front): On some LG over-the-range models, you can remove the control panel assembly from the front after removing the top grille and one or two mounting screws. With power disconnected, carefully reseat the flat ribbon cable from the keypad to the control board. Do not bend or crease the ribbon.
• Test operation: Restore power and test several keys. If F1 returns immediately or certain keys do not respond, the keypad or control panel assembly is likely defective.

When to call a technician: If reseating the ribbon and cleaning the keypad does not clear F1, the membrane keypad or the entire control panel assembly usually must be replaced. This requires disassembly near high-voltage components; have a qualified technician replace the keypad/control panel and test the unit for safety.