LG microwave fan and light staying on: separate cooling from a fault

Use only checks that match the exact appliance or system and its official guide.

1. Check 1

   Stop cooking and identify the illuminated item: the cooktop or night light under an over-the-range unit, the oven-cavity light, or only a display indicator. Read the exact LG model number before using a button sequence.

   Expected result: The light is a separate cooktop/night light, the cavity light follows door operation, or the model and light type cannot be confirmed.

   Next step: If the panel matches LMV1680WW/BB/ST, use that manual's LIGHT control sequence; otherwise use the exact model manual. If the light type or model is uncertain, stop button experiments and arrange service if it remains on.

   Why this matters

   Why: The light and vent controls are not necessarily the same circuit or function. The opened LMV1680 manual documents separate VENT and LIGHT controls, so a light complaint should not be diagnosed as a fan-switch failure without identifying the light.

2. Check 2

   Check the display and recent use for an automatic cooling state. After cooking or extended operation, leave the door and controls alone while the indicated cooling cycle completes; do not force the fan off during that state.

   Expected result: Cool is displayed, the fan has run only briefly after cooking, or the fan continues well beyond the documented model behavior with no cooling indication.

   Next step: If the fan stops when cooling ends, no repair is indicated. If it continues with no cooking, no heat, and no cooling indication, continue only with external control checks and prepare for qualified service.

   Why this matters

   Why: LG says Auto Cooling can run temporarily after cooking and show Cool; the LMV1680 manual gives a separate heat-protection example of a vent fan that may run up to an hour and cannot be manually stopped. A persistent fan outside those conditions needs diagnosis.

3. Check 3

   If the microwave is an over-the-range or ThinQ-synced installation, stop the cooktop and check whether a hood or microwave-sync mode is active. Use only the exact appliance or app instructions to change that mode.

   Expected result: The fan and light turn on with active cooktop use or a documented sync mode, or they remain on after the cooktop and sync state are off.

   Next step: If the pair follows the active cooktop or sync state, use the matching controls or app setting. If it remains on with no active trigger, do not infer a failed relay; move to the power-isolation and service boundary.

   Why this matters

   Why: LG describes Hood and Microwave Sync as automatically turning the fan and light on while cooking and off when done. A fan/light pair that remains on after the documented trigger ends is different from normal synchronized operation.

4. Check 4

   When the fan and light stay on after cooking, cooling, and any documented sync state have ended, stop using the microwave. If the plug or breaker is safely accessible, disconnect power without removing a panel, then arrange qualified microwave service.

   Expected result: Power isolation stops the fan/light, or the fan/light behavior persists, the microwave is damaged, the door does not close correctly, or a safety interlock is involved.

   Next step: Leave the unit disconnected and do not restore power for repeated tests if the behavior returns. Do not open the cabinet, defeat an interlock, or test internal wiring; provide the model, light type, display state, and timing to qualified service.

   Why this matters

   Why: A persistent paired output is outside the normal user-control and cooling conditions established by the opened LG materials. Microwave door, interlock, control, and high-voltage work is a professional boundary.