Sharp oven shows F20 and may not start microwave or combination functions (for Sharp microwave-oven combos).

Critical safety warning: Microwave components store lethal high voltage even when unplugged. Do not attempt internal microwave repairs unless you are trained and equipped. The following is for diagnostic awareness only.

• Basic checks only: Verify that the door closes properly and that the cavity light and fan operate when starting a microwave cycle. If they do not, a door switch or control issue is likely.
• Door switches: Door interlock switches are a common cause. A technician can test and replace faulty switches.
• Internal HV components: Magnetron, transformer, capacitor, and diode should only be tested and replaced by a qualified technician due to high-voltage risk.
• Do not bypass safety devices: Never bypass door switches or fuses to "test" the unit.