Microwave does not heat and displays F4 shortly after starting.

Critical safety warning: High-voltage components inside a microwave can be lethal. Do not touch the magnetron, capacitor, or transformer. If you are not trained, contact a professional.

• Step 1 – Check ventilation: Ensure the microwave has adequate clearance around vents and that intake/exhaust grills are not blocked by cabinets, dust, or grease filters.
• Step 2 – Clean filters (for built-in/hood models): Remove and clean grease filters and check any charcoal filters. Clogged filters can cause overheating.
• Step 3 – Power reset: Unplug for 15 minutes to allow thermal cut-outs to cool and reset (if they are self-resetting types). Reconnect and test.
• Step 4 – Inspect cooling fan operation: Start a short heating cycle and listen for the internal cooling fan. If the fan does not run, the magnetron will overheat and trigger F4.
• Step 5 – Professional checks: A technician should test continuity of the magnetron thermostat and cavity thermal cut-outs, and verify the cooling fan motor and wiring.
• Step 6 – Replace faulty parts: Replace any open thermal cut-out, defective thermostat, or failed cooling fan with genuine Bauknecht parts.