When a residential microwave oven runs normally for approximately three seconds and then abruptly cuts off power, the premature shutdown is safety-engineered by the control board in response to a hardware fault. The most frequent cause is a failing door interlock microswitch that opens its electrical contacts under operating vibrations, signaling the control system that the door has been opened. Another common cause is a tripped thermal protector due to a blocked cooling fan or failing magnetron circuit. You can troubleshoot shutdown faults safely by unplugging the unit and inspecting the door assembly. You can verify that your corrective action is successful when replacing a faulty door microswitch, securing door latches, or clearing blocked ventilation grilles allows the microwave to run through a full heating cycle.
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