Many small household appliances or circuit boards have devices called diodes placed on the circuit. A diode acts as a gateway to keep electrical current traveling in one direction. If a diode functions improperly, it will either allow complete flow of alternating current (AC) or no current at all. One of the main functions of a diode is to rectify AC into direct current (DC). A malfunctioning diode can ruin your appliances; you can test a diode's functionality with the resistance measurement function of a multimeter.
Plug the red lead of your multimeter into the socket marked "Voltage+."
Plug the black lead into the socket marked "COM."
Turn on the device you want to test.
Locate the diode you want to test. The diode is a small cylindrical piece with a band on one end. This band indicates the directional bias. One end will be the anode, or positive charge, and the other end will be the cathode, or negative charge.
Set the meter to measure resistance, indicated by the Greek letter omega and touch the leads to each end of the diode. You should get a low resistance measurement when the positive lead is touched to the anode and the negative to the cathode.
Switch the leads around. You should get a high reading, or "OL," indicating overload.
Connect the leads to the meter: red to "Volt+" and black to "COM."
Set the meter to the diode function. This is indicated with a cross with an arrow pointed toward the center.
Touch the leads to the ends of the diode. If you have the leads connected properly -- red lead to the anode, black to the cathode -- you should get a voltage measurement in millivolts.
Switch the position of the leads. You should get an undefined reading "OL" because the diode is not intended to allow voltage in that direction.