Microwave displays E4 and will not heat or start any program.

Critical safety warning: The inverter and high-voltage circuits in a microwave operate at lethal voltages. Do not attempt to service the inverter, magnetron, or high-voltage wiring unless you are a trained technician with proper tools and procedures to discharge the capacitor.

• Step 1 – Basic checks: Unplug the microwave. Inspect the power cord and outlet. Confirm that other functions (display, interior light, turntable) work when attempting to start a cycle.
• Step 2 – Listen for abnormal sounds: If the unit clicks and immediately stops with E4, the inverter is detecting a fault (often a shorted magnetron).
• Step 3 – Visual inspection (no contact): With the cabinet removed (unplugged), visually inspect the inverter PCB for burnt components, cracked solder joints, or melted connectors. Do not touch the board or HV wiring.
• Step 4 – Professional service: Because of the high risk, replacement of the inverter board or magnetron should be done by an authorized Bomann service center. Provide them with the exact model number and E4 code description.

If you are not qualified, limit your work to external checks and do not operate the microwave with the cabinet removed.