Microwave does not heat, runs normally (light and fan on), and displays E03 after a few seconds.

Safety Warning: The high-voltage section of a microwave can exceed 4,000 volts. Only qualified technicians should test or replace high-voltage components.

User-level steps:

• Unplug the microwave immediately when E03 appears.
• Confirm that you are using standard cookware (microwave-safe glass or plastic) and not metal or dishes with metallic trim.
• Try heating a cup of water for 1–2 minutes. If there is no heating and E03 appears, internal service is required.

Technician-level outline (do not attempt if untrained):

• Remove the cabinet and discharge the high-voltage capacitor.
• Test the high-voltage diode with a multimeter capable of high-voltage diode testing; replace if shorted or open.
• Check the high-voltage capacitor for shorts or opens using a capacitance meter.
• Measure primary and secondary windings of the high-voltage transformer for continuity and proper resistance.
• Test the magnetron for shorts to ground and continuity across terminals.

Because of the danger involved, most homeowners should not attempt this repair and should contact an authorized Insignia service provider.