What a diode test tells you
A diode is a small electronic component that lets electricity flow in one direction only. When you test a diode with a multimeter, you are checking whether it still works the way it should — either it conducts current in the forward direction and blocks it in reverse, or something inside it has failed. A working diode shows low resistance in one direction and very high resistance (or no reading) in the other. A dead diode shows high resistance both ways, or conducts both ways when it should not.
Most multimeters have a dedicated diode test setting, which is faster and more accurate than using the resistance setting alone. The diode setting sends a small test current through the component and displays the voltage drop — usually between 0.4 and 0.7 volts for a working silicon diode — rather than a resistance number. This voltage drop is the real signal that the diode is functioning.
Key Takeaways
- Set your multimeter to the diode symbol (⏚) setting, not the resistance setting, for the most accurate test.
- A working diode shows a voltage drop of 0.4 to 0.7 volts in the forward direction and no reading or "OL" (overload) in the reverse direction.
- You must remove the diode from the circuit board or disconnect it from other components before testing, or you will get false readings.
- The red probe always connects to the positive end of the diode (the anode) and the black probe to the negative end (the cathode) to test forward bias.
Prepare the diode and multimeter
Before you test, remove the diode from the circuit or at least disconnect one of its leads. A diode still soldered to a board or connected to other components will give you a false reading because the multimeter's test current will take alternate paths through the circuit. If you cannot desolder it safely, test it in place as a last resort — just know that the results may not be reliable.
Set your multimeter to the diode test mode. Look for the symbol that looks like a triangle with a line across it (⏚), usually found on the dial near the resistance settings. If your multimeter does not have a dedicated diode setting, you can use the resistance (ohms) setting instead, though it will be less precise. Do not use the voltage setting — that will not test the diode at all.
Test the diode in the forward direction
The forward direction is when current flows through the diode the way it is designed to work. To test this, connect the red probe to the positive end of the diode (called the anode) and the black probe to the negative end (called the cathode). The cathode is usually marked with a stripe or band around the diode body.
Look at the multimeter display. A working diode will show a voltage drop between 0.4 and 0.7 volts — most commonly around 0.5 to 0.6 volts for a standard silicon diode. Some multimeters display this as a decimal number like "0.65V". If the display shows "OL" (overload) or no reading at all, the diode is likely dead or the connection is loose. Check that both probes are making firm contact with the diode leads.
Test the diode in the reverse direction
Now reverse the probes to test the reverse direction. Connect the black probe to the anode (positive end) and the red probe to the cathode (negative end). This is the direction the diode should block.
A working diode will show "OL" (overload), "1" on the left side of the display, or simply no reading at all. This means the diode is successfully blocking current in the reverse direction, which is what you want. If the display shows a voltage drop similar to the forward test (0.4 to 0.7 volts), the diode has failed and is conducting both ways — it should be replaced.
Interpret the results
A healthy diode shows a clear difference between the two directions: a voltage drop in one direction and a block in the other. If you see a voltage drop in both directions, or no reading in both directions, the diode is bad. If the readings are very close to each other (within 0.1 volts), the diode is also likely failing.
Keep in mind that different types of diodes have different forward voltage drops. A standard silicon diode drops 0.5 to 0.7 volts, but a germanium diode drops only 0.2 to 0.3 volts, and a Schottky diode drops 0.2 to 0.4 volts. If you know what type of diode you are testing, check the datasheet to see what voltage drop is normal for that specific component. If you do not know the type, a reading between 0.3 and 0.7 volts in the forward direction is usually a sign the diode is working.
Common mistakes that give false results
The most common error is testing a diode that is still soldered into a circuit. The multimeter's test current will flow through other components on the board, and you will see a reading that does not reflect the diode's actual condition. Always remove or isolate the diode first if you can.
Another mistake is reversing the probe connections and then thinking the diode is bad because it shows "OL" in what you thought was the forward direction. Double-check which end of the diode is the cathode (the striped end) before you start. If you are unsure, test both ways and look for the direction that gives you a voltage drop — that is the forward direction. Loose probe contact is also common. Make sure both probes are pressing firmly against the diode leads. If the display flickers or shows an unstable reading, adjust the probe position slightly until you get a steady number.
When to use resistance mode instead
If your multimeter does not have a diode test setting, switch to the resistance (ohms) setting and use the lowest ohms range available. Connect the red probe to the anode and black to the cathode. A working diode will show low resistance — typically a few hundred ohms or less. Then reverse the probes. A working diode will show very high resistance or "OL" in the reverse direction.
The resistance readings will be less precise than the voltage drop method, but they will still tell you whether the diode is conducting or blocked. This approach works well for a quick test when you do not have access to the diode setting, though you should switch to the diode setting if your multimeter has one and you need a more definitive answer.
Frequently Asked Questions
What if the diode shows the same reading both ways?
If the diode shows a voltage drop in both directions, it has failed internally and is conducting electricity when it should be blocking it in one direction. The diode should be replaced. If it shows no reading or "OL" both ways, the diode is open (broken) and not conducting at all, which also means it needs replacement.
Can I test a diode while it is still soldered to the board?
You can try, but the results will often be wrong because current flows through other components connected to the diode. If you must test in place, disconnect at least one lead from the board first. If that is not possible, treat any reading as unreliable and desolder the diode for a proper test.
Why does my multimeter show different voltage drops for the same diode each time?
Small variations (within 0.1 volts) are normal and depend on the multimeter's internal battery voltage and temperature. If the readings jump around wildly or change by more than 0.2 volts between tests, the diode may be failing or the probe contact may be loose. Clean the probe tips and try again with firm contact.
What is the difference between the diode symbol setting and the resistance setting?
The diode setting sends a precise test current and measures the voltage drop, which is the standard way to test diodes. The resistance setting measures how much the diode opposes current flow, which works but is less accurate. If your multimeter has a diode setting, use it — the voltage drop reading is more reliable than a resistance reading.
How do I know which end of the diode is the cathode?
The cathode is marked with a stripe or band around the diode body. The anode is the unmarked end. If the stripe is worn off or hard to see, you can test both ways — the direction that shows a voltage drop (not "OL") is the forward direction, which means the probe on the unmarked end is touching the anode.