Microwave displays F3 and will not start or stops with an error shortly after beginning a cook cycle.

Safety: Disconnect power before accessing internal components. Do not touch high-voltage sections.

• Locate the sensor: On Monogram over-the-range and built-in microwaves with convection or sensor cooking, the oven temperature sensor (thermistor) is typically mounted inside the cavity or behind an interior panel.
• Visual inspection: Check the sensor and its wiring for obvious damage, burns, or broken insulation.
• Resistance test: With power disconnected, remove the sensor connector from the harness. Use a multimeter to measure resistance across the sensor leads. Compare to the Monogram service specifications (commonly around 1000–1100 Ω at room temperature for many thermistors). A reading of open (OL) or near 0 Ω indicates a bad sensor.
• Check harness and connections: Inspect the wiring from the sensor to the control board for pinched, broken, or burned wires. Reseat the connector at the control board.
• Replace sensor: If resistance is out of spec, replace the oven temperature sensor / thermistor with the correct Monogram part. Secure it in the same location and route the wiring away from hot or sharp surfaces.
• Control board check: If the sensor tests good and wiring is intact, the electronic control board may have a failed sensor circuit and will need replacement.