Unplug the microwave from the wall outlet.
Remove the screws on the microwave's outer casing; the type and number of screws varies by model. Lift the outer casing off of the microwave to reveal its internal components.
Discharge the high-voltage capacitor using a metal screwdriver with a well-insulated handle. Touch the blade of the screwdriver to both capacitor terminals at the same time; this may cause a small spark or loud popping sound.
Look to the left of the electronic control panel to locate the door opening mechanism. Find the door switch at the top of the mechanism, just below the upper door latch hook. Disconnect the door switch wiring; use a pair of needle-nose pliers to pull the connectors off of the switch terminals.
Turn on the digital multimeter and set it to measure Rx1 resistance. Touch the multimeter leads to the door switch terminals; you should receive a reading of infinity. Continue to hold the multimeter leads to the terminals and press the actuator on the door switch; the meter should register a reading of zero ohms. Replace the door switch if the multimeter produces readings other than infinity or zero.
Remove the screws holding the door switch to the door open mechanism. Discard the faulty switch, but reserve the mounting screws. Hold the new door switch to the mechanism and replace the reserved screws. Reattach the wires to the door switch terminals.