What a diode does and why you might need to test one
A diode is a small electronic component that lets electricity flow in one direction only. When it works correctly, it blocks current going the wrong way. When it fails, it either blocks current in both directions or lets it through in both directions — either way, your circuit stops working the way it should.
You test a diode when a device stops working and you suspect the diode is the problem, or when you want to confirm a diode is good before you solder it into a circuit. A multimeter set to diode mode tells you whether the component is working, shorted (conducting both ways), or open (conducting neither way).
The test takes about 30 seconds per diode and requires only a multimeter with a diode setting — most digital multimeters have one. You do not need to remove the diode from the circuit in most cases, though removing it gives you a clearer answer.
Key Takeaways
- Set your multimeter to diode mode (usually marked with a diode symbol that looks like an arrow with a line) before you start testing.
- A working diode shows a voltage reading between 0.4 and 0.7 volts in one direction and displays "OL" (overload) or infinity in the other direction.
- If the diode shows the same reading in both directions or shows "OL" in both directions, it has failed and needs to be replaced.
- Remove the diode from the circuit before testing if you want the most reliable result, because other components can interfere with the reading.
Setting up your multimeter for diode testing
Turn off power to the circuit or device you are testing. Even though a multimeter's diode mode uses only a tiny amount of current, you want no power running through the component while you test it.
Locate the diode symbol on your multimeter's dial or display. On analog multimeters, it looks like a small arrow pointing at a line. On digital multimeters, it is usually marked with the same symbol and grouped near the resistance or continuity settings. Turn the dial or press the button to select diode mode.
If your multimeter does not have a dedicated diode mode, you can use the resistance (ohms) setting instead, though it is less reliable. Diode mode is designed specifically for this test and gives clearer readings.
How to connect the probes and read the result
Hold the diode or touch the multimeter probes to the diode's leads. A diode has two ends: the anode (positive end, usually marked with a red or unmarked band) and the cathode (negative end, usually marked with a black or silver band). The band is the key — it points toward the cathode.
Touch the red probe to the anode and the black probe to the cathode. A working diode displays a voltage reading between 0.4 and 0.7 volts. This is the forward voltage drop — the amount of voltage the diode uses up when current flows through it in the correct direction. The exact number depends on the diode type, but anything in that range means the diode is good.
Now reverse the probes: touch the red probe to the cathode and the black probe to the anode. A working diode displays "OL" (overload), "∞" (infinity), or "1" on the display. This means the diode is blocking current in the reverse direction, which is exactly what it should do. If you see a voltage reading instead of "OL", the diode has failed.
What each test result means
A good diode shows a voltage reading (0.4 to 0.7 volts) in one direction and "OL" or infinity in the other. The direction matters — you should see the voltage reading when the red probe touches the anode. If the readings are reversed, you have the diode oriented backward, so flip it and test again.
A shorted diode shows a voltage reading in both directions, or shows "OL" in both directions. Either way, it is not doing its job. A shorted diode conducts electricity both ways instead of blocking one direction. This usually means the component has failed internally and needs to be replaced.
An open diode shows "OL" or infinity in both directions. This means the diode is not conducting in either direction — the internal connection has broken. Like a shorted diode, an open diode has failed and must be replaced.
If you see a very low voltage reading (close to 0 volts) in the forward direction, the diode may be damaged or the multimeter battery may be low. Try testing a diode you know is good to compare. If that one also reads very low, replace the multimeter battery.
Testing a diode that is still soldered in the circuit
You can test a diode without removing it from the circuit, but other components nearby can interfere with the reading. If the diode is soldered to a circuit board, touch the probes to the diode's leads directly — do not touch the solder joints or nearby traces, because that can give you a false reading.
If you get a confusing result — for example, a voltage reading in both directions — the circuit board itself may be affecting the test. In that case, remove the diode and test it again in isolation. Desoldering takes a few minutes with a soldering iron or desoldering pump, and it gives you a definitive answer.
If you are not comfortable desoldering, you can also test the diode by measuring the voltage across it while the circuit is powered on. A working diode in a powered circuit shows a small voltage drop in the forward direction and nearly full supply voltage in the reverse direction. This is a more advanced test and requires understanding the circuit, so stick with the multimeter diode mode test if you are unsure.
Common mistakes that give you wrong answers
The most common mistake is testing with the probes reversed. If you touch the red probe to the cathode instead of the anode, you will see "OL" in the forward direction and a voltage reading in the reverse direction — the opposite of what you expect. This does not mean the diode is bad; it means you have the probes backward. Flip them and test again.
Another mistake is testing a diode that is still powered. If the circuit is on, the power supply can override the multimeter's test signal and give you a false reading. Always turn off power before you test.
Testing a diode while it is still soldered to a circuit board can also confuse the result, because the surrounding components conduct electricity and interfere with the reading. If you get an unexpected result, remove the diode and test it alone.
Finally, do not confuse a low voltage reading with a bad diode. Some diodes have a higher forward voltage drop than others — LEDs, for example, can show 1.5 to 3 volts instead of 0.4 to 0.7. If you are testing a special type of diode, look up its datasheet to see what voltage drop is normal.
When to replace a diode instead of testing it
If the diode fails the test, you need to replace it. Diodes are inexpensive — usually between 10 cents and a few dollars each — and they cannot be repaired. Desoldering the old one and soldering in a new one takes 5 to 10 minutes if you have a soldering iron.
Make sure you install the new diode with the correct orientation. The band on the new diode must point the same direction as the band on the old one. If you install it backward, it will not work and may damage other parts of the circuit.
If you are replacing a diode in a circuit board and you are not sure which direction it goes, look at the circuit diagram or the silkscreen markings on the board. Most boards have a small line printed next to the diode pad to show where the band should be.
Frequently Asked Questions
Can I test a diode with just the resistance setting on my multimeter?
Yes, but diode mode is more reliable. In resistance mode, a working diode shows a low resistance in one direction (usually a few hundred ohms) and very high resistance in the other direction. The exact numbers vary by multimeter, so diode mode is clearer because it shows you a voltage reading instead.
What if my multimeter does not have a diode setting?
Use the resistance (ohms) setting instead. A working diode shows low resistance in one direction and very high resistance in the other. If the resistance is the same in both directions, or very high in both directions, the diode has failed. This method is less precise but still tells you whether the diode is good or bad.
Do I have to remove the diode from the circuit to test it?
No, but you will get a clearer answer if you do. Testing a soldered diode can give false readings because other components on the board conduct electricity. If your first test result seems wrong, remove the diode and test it again in isolation.
What voltage should a good diode show?
Between 0.4 and 0.7 volts in the forward direction. Standard silicon diodes show around 0.6 volts. LEDs and some specialty diodes show higher voltages — up to 2 or 3 volts. If you are testing a specific type of diode, check its datasheet to see what voltage is normal.
Can a diode fail partially, or does it either work or not work?
A diode either works or it does not. It cannot conduct partially. If it fails, it either shorts (conducts both ways) or opens (conducts neither way). You will see one of these two failure modes in your test — there is no in-between.