F-1 — Main Control Board Fault

How to fix it

1. Unplug the microwave for at least 60 seconds, then restart — clears transient faults.
2. If over-the-range, remove from mounting plate or pull from countertop and remove the top/back grille for access.
3. Locate the oven thermistor (small probe in the cavity wall or top duct) and unplug its connector.
4. Measure resistance across the thermistor leads with a multimeter at room temperature — should read roughly 10K ohms. Open (OL) means the sensor is bad.
5. Inspect the harness from sensor to control board for chafe, pinch, or melted insulation.
6. Replace the thermistor if out of range or open; reconnect firmly.
7. If sensor reads correct and harness is good, replace the main control (PCB) board.
8. Reassemble, restore power, and run a Sensor Reheat test cycle to confirm.

Why it happens

Typical signs: F-1 or F1 on display · Sensor Cook / Auto Cook cycles fail immediately · Microwave still runs in manual time-cook mode · Unit beeps and stops mid-cycle

• Failed thermistor The sensor probe (typically 10K-ohm NTC) has opened. Most common cause.
• Disconnected sensor harness Plug at the thermistor or at the control board has vibrated loose.
• Pinched / broken sensor wire Wire damaged where it routes past the cavity vent.
• Main control board fault Sensor input circuit on the PCB has failed — replace only after ruling out the probe.

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 main control board Common replacement
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