Why does my microwave only work for 3 seconds?

When a microwave oven runs for approximately three seconds and then shuts off abruptly, the premature shutdown is invariably triggered by an active safety interlock or thermal protection protocol. The primary cause is a failing or loose door interlock microswitch that loses electrical contact under operating vibrations, causing the control board to immediately cut power as a safety precaution against microwave radiation leaks. Another potential cause is an overheated magnetron or transformer tripping a thermal cutoff thermostat. You can troubleshoot shutdown faults safely by unplugging the appliance and inspecting the door latch assembly. You can verify that your corrective action is successful when adjusting misaligned door latches, clearing blocked cooling vents, or replacing a faulty door microswitch allows the microwave to run through a complete heating cycle.