A diode works one way only — electricity flows through it in one direction and gets blocked in the other

To check whether a diode is working, you use a multimeter — a handheld device that measures electrical properties. Most multimeters have a diode-testing mode (often marked with a diode symbol that looks like an arrow with a line across it). When you touch the multimeter's probes to each end of the diode, the meter tells you whether current can flow through it in that direction. A working diode will show a voltage reading (usually between 0.4 and 0.7 volts) when tested one way, and either no reading or "OL" (open line) when tested the other way. If it shows the same reading both directions, or no reading either way, the diode is damaged.

This matters because diodes are everywhere — in power supplies, LED lights, solar panels, and circuits that need to let electricity move in only one direction. When one fails, the whole device often stops working. Testing takes about 30 seconds and costs nothing if you already own a multimeter.

Key Takeaways

  • A working diode shows a voltage reading (typically 0.4 to 0.7 volts) when the red probe touches the positive end and the black probe touches the negative end.
  • When you reverse the probes, a working diode shows no reading or "OL" — this proves it only lets electricity flow one direction.
  • If the diode shows the same reading both ways, or shows no reading either way, it is damaged and needs replacement.
  • You need a multimeter set to diode mode, which most basic multimeters include — you do not need special equipment.

What the two ends of a diode are and how to tell them apart

A diode has two ends: the anode (positive end) and the cathode (negative end). On a physical diode, the cathode is marked with a stripe or band around one end — that is the end you need to know. The unmarked end is the anode. Current flows from anode to cathode inside the diode, so the diode only works when the anode connects to positive voltage and the cathode connects to negative or ground.

If you cannot see the stripe clearly (it may be faded or the diode may be very small), you can test both directions with the multimeter and use the results to figure out which end is which. The end that shows a voltage reading when the red probe touches it is the anode.

How to set up your multimeter for diode testing

Turn the multimeter's dial to the diode symbol. On most multimeters, this is a separate position on the dial, often grouped near the resistance (ohms) settings. If your multimeter does not have a dedicated diode symbol, some models include diode testing within the resistance mode — check your multimeter's manual if you are unsure.

Make sure the red probe is plugged into the port marked "VΩmA" or similar (the main measurement port), and the black probe is in the "COM" or common port. These are the standard probe positions for almost all multimeter measurements. If your probes are in the wrong ports, the meter will not give you a reading.

Testing the diode in the forward direction

Touch the red probe to the anode (the unmarked end of the diode) and the black probe to the cathode (the striped end). The multimeter should display a voltage reading, usually between 0.4 and 0.7 volts. The exact number depends on the type of diode — silicon diodes typically read around 0.6 volts, while germanium diodes read closer to 0.3 volts. What matters is that you see a number, not that it matches a specific value.

If the display shows "OL" or no reading at all when you test in this direction, the diode is likely damaged. A working diode should always show some voltage when tested forward.

Testing the diode in the reverse direction

Now reverse the probes: touch the black probe to the anode and the red probe to the cathode. A working diode will show "OL" (open line) or no reading at all. This is the correct behavior — the diode is blocking current in this direction, which is exactly what it is supposed to do.

If the diode shows a voltage reading in both directions, it is shorted and needs to be replaced. If it shows no reading in both directions, it is open (broken) and also needs replacement. Only a working diode shows a reading one way and no reading the other way.

What to do if the diode is in a circuit

If the diode is still soldered to a circuit board, you should desolder it first before testing. Testing a diode while it is connected to other components can give false readings because the surrounding circuit provides alternate paths for current. Desoldering takes a soldering iron and a few minutes, but it is the only way to get an accurate test.

If you do not have soldering equipment and cannot remove the diode, you can still test it in place, but understand that a "bad" reading might mean the diode is damaged or might mean the circuit around it is interfering with the test. In that case, you would need to either desolder it for a clean test or replace it and see if the device works.

Common reasons a diode test fails

The most common reason a diode appears to fail is that the multimeter is not set to diode mode — if you test in resistance mode instead, you will get confusing readings. Double-check that the dial is pointing to the diode symbol before you conclude the diode is bad.

The second most common reason is testing a diode that is still soldered into a circuit. The surrounding components create paths for current that bypass the diode, so the test does not show the diode's true behavior. If you get unexpected results, desolder the diode and test it in isolation.

A third possibility is that the diode is the right type for the circuit but the multimeter's test voltage is too low to forward-bias it. Some specialized diodes (like Schottky diodes) have different voltage thresholds. If you see no reading in the forward direction but the diode looks intact and is not in a circuit, it may be a type your multimeter cannot test reliably — in that case, you would need a more advanced meter or a datasheet for that specific diode.

Frequently Asked Questions

What if my multimeter does not have a diode mode?

You can test a diode using the resistance (ohms) mode instead, though it is less reliable. Set the multimeter to the lowest ohms range. A working diode will show very low resistance one way (a few hundred ohms or less) and very high resistance the other way (often "OL"). Reverse the probes and repeat. The pattern should flip — low one way, high the other.

Can I test a diode while it is powered on?

No. Always power off the circuit and discharge any capacitors before testing. Testing a powered diode can damage the multimeter or give false readings because the circuit voltage interferes with the meter's test voltage.

What does "OL" mean on the multimeter display?

OL stands for "open line" and means the multimeter cannot measure a complete circuit — in other words, no current is flowing. When testing a diode in reverse, OL is the correct result and shows the diode is working properly by blocking current.

Do all diodes show the same voltage reading?

No. Different types of diodes have different forward voltage drops. Silicon diodes typically read 0.6 to 0.7 volts, germanium diodes read 0.2 to 0.3 volts, and Schottky diodes read 0.15 to 0.45 volts. What matters is that you see a reading in the forward direction and no reading in reverse, not the exact number.

Can a diode fail partially?

Rarely. A diode usually either works (shows the correct forward and reverse readings) or is completely failed (shows the same reading both ways, or no reading either way). A partially failed diode is uncommon, but if you see a very low reading in reverse (instead of OL), the diode is leaking current and should be replaced.