A diode is an electronic component that allows electric current to flow in one direction. If the current reverses direction, the diode blocks the flow. The 1N4007 diode is a small, general purpose device that can handle currents up to 1 ampere and voltages up to 1000 volts. Like any electronic part, a diode can fail. It may fail to open and conduct current in either direction, or it might short and conduct in both directions. The diode may also leak excessively in the reverse direction. You can test a 1N4007 diode by using a multimeter, a battery and a low-value resistor.
Examine the 1N4007 diode carefully and note that one end of the device has a white band. This indicates the diode's cathode end. The other end is the anode.
Turn the multimeter on and set its function knob so that the meter reads resistance. Make sure that the black probe wire is plugged into the common, or negative, socket on the meter and that the red probe is plugged into the positive socket.
Touch the multimeter's black probe to the cathode lead of the 1N4007. This is the side with the band or stripe. Touch the red probe to the diode's anode lead. You may find it convenient to wrap the diode's leads around the probe tips. Avoid touching the probe tips or leads with your fingers, as the resistance of your body will distort the meter's reading. The meter should read 0 ohms or a very low value, such as 1 ohm. If you read a high resistance, the diode has broken open internally, so discard it.
Reverse the probe wires on the diode so that the black one is now touching the anode lead and the red probe is on the cathode. The meter should now read a very high resistance. The meter may read "OL" for overload or something similar to indicate a resistance too high for the meter to read. If you read a very low resistance, the diode is shorted and therefore bad.
Remove the diode from the meter probe tips. Turn the meter's selector knob so it reads DC milliamps (thousandths of an amp) or microamps (millionths of an amp). Wrap or crimp one lead of the 470 ohm resistor to the AA battery clip's positive terminal. Connect the diode's cathode lead to the resistor's free lead. Wrap the anode around the multimeter's positive probe tip. Connect the meter's negative probe tip to the battery clip's negative terminal. Snap a fresh AA battery into the clip. You should see a current reading of no more than 5 microamps. If the meter measures significantly more than this, the diode is leaky and should be discarded..