Microwave will not start or stops with F3 on the display, often when using sensor cook or reheat functions.

Safety first: Unplug the microwave. Internal components can be hazardous; avoid the high-voltage section.

• Reset the unit: Disconnect power for 10 minutes, then reconnect and test a basic timed cook (e.g., 30 seconds). If F3 appears only on sensor modes, the sensor is suspect.
• Locate the sensor: On many Hotpoint models, the humidity or temperature sensor is mounted on the side or top of the cavity behind a small grille or cover.
• Inspect the sensor and wiring: Remove the outer cabinet (screws at back and sides). Locate the sensor and follow its wiring harness to the control board. Check for pinched, burnt, or broken wires and secure connections.
• Test or replace sensor: If you have a multimeter and the service manual, check the sensor resistance against specifications. If out of range or shorted, replace the cavity temperature/humidity sensor.
• Control board: If a known-good sensor still results in F3, the main control board sensor circuit may be defective and require replacement.