Microwave shows E15 and makes a loud humming noise without heating.

Critical safety warning: The high-voltage section can be lethal. Only trained persons should work on these parts. Always unplug and discharge the capacitor properly.

• Do not continue to run the unit: Prolonged operation with a loud hum and no heating can damage the transformer.
• Visual inspection: Remove the cover and inspect the HV capacitor and diode for burn marks, cracks, or leakage.
• Test HV capacitor and diode: Using appropriate test methods and a meter rated for the task, check the capacitor for shorts or opens and the diode for correct one-way conduction. Replace any faulty components with Telefunken-compatible parts.
• Check transformer: If the hum is extremely loud and components test good, the HV transformer may be failing and should be replaced by a professional.