A diode is an electronic device that permits electric current to flow through it in only one direction. Like a check valve in a plumbing system, it shuts off the flow of electricity in the reverse direction. A real-world diode, however, will leak a tiny bit of backward current. Usually this is not enough to affect the circuit in which it works. A defective diode will leak excessive current. You can't fix a leaky diode. It must be replaced.
Press the 9-volt battery into its clip. Note that the clip has a red wire and a black wire.
Turn the multimeter on and set its selector knob to read direct current (DC) milliamps, or thousandths of an amp. Plug the black meter probe wire into the meter's common jack. The meter may have a special jack for reading DC milliamps. If so, plug the red wire into that jack. Otherwise, plug the red wire into the meter's positive jack.
Examine the diode. Note it has a stripe or band around one end. That end is the cathode. The other end of the diode is the anode. Wrap the bare end of the 9-volt battery clip's red wire around the one lead of the 470 ohm resistor. Wrap the other resistor lead around the diode's cathode lead. Wrap the bare end of the battery clip's black wire around the bare tip of the meter's black (negative) probe. Wrap the diode's anode lead around the meter's red (positive) probe.
Read the current displayed on the meter. It may read zero, or it may read a very small number, such as 10 microamps, or millionths of an amp. If the meter reads much more than 5 milliamps, the diode is excessively leaky.