Testing a Diode

• 1). Set the control dial on your multimeter to the "diode" setting. Most digital multimeters have a diode test setting indicated by the schematic symbol for a diode, which is an arrow pointing at a line.
  
  
• 2). Connect the red test lead of the multimeter to the anode of the diode, and the black lead to the cathode. The cathode end of a diode is indicated by a band on the case. If the diode is working, the meter will display the forward voltage, often about 0.7 volts.
  
  
• 3). Reverse the test leads. The meter should read "OL," indicating an open circuit. Any other reading means the diode has failed.
  
  

Testing a Transistor

• 1). Set your meter to "diode" mode.
  
  
• 2). Connect the red lead to the base of the transistor and the black lead to the emitter. If it's an NPN transistor, the meter should show the forward voltage drop of the transistor, usually a value between about 0.5 and 1 volt. With a PNP transistor, the meter should read "OL," indicating an open circuit. An open circuit means that there's no electrical connection.
  
  
• 3). Switch the black test lead to the collector and leave the red lead connected to the base. The readings should be identical to the previous test.
  
  
• 4). Reverse the test leads so that the red lead is connected to the collector and the black lead is connected to the base of the transistor. The readings should be the reverse of the previous test.
  
  
• 5). Connect the red test lead to the emitter of the transistor, with the black test lead still connected to the base. If it's an NPN transistor, the meter should read that it's an open circuit. If you're testing a PNP transistor, the meter should show the forward voltage drop.
  
  
• 6). Connect one of the test leads to the collector of the transistor and the other to the emitter. It doesn't matter which lead you connect to which. The meter should indicate an open circuit.