How to test a diode on your multimeter
Most multimeters have a dedicated diode test mode — usually marked with a diode symbol that looks like a triangle with a line through it. To test a diode, switch your multimeter to that mode, touch the red probe to the positive end of the diode (the anode) and the black probe to the negative end (the cathode), then read the voltage drop on the display. A working diode typically shows a reading between 0.4 and 0.7 volts. If you reverse the probes, a good diode should show no reading or "OL" (open line), meaning it's blocking current in that direction.
The diode test mode works by sending a small current through the component and measuring the voltage drop. This is different from using the resistance setting, which can sometimes give misleading results because diodes are designed to conduct in only one direction. Using the dedicated diode mode gives you a clearer picture of whether the diode is working as it should.
Key Takeaways
- Switch your multimeter to diode test mode — the symbol looks like a triangle with a line through it — before you begin testing.
- A working diode shows a voltage drop between 0.4 and 0.7 volts when the red probe touches the anode and the black probe touches the cathode.
- Reverse the probes to confirm the diode blocks current in the opposite direction; you should see no reading or "OL" on the display.
- If the diode shows the same reading in both directions or no reading in either direction, it is likely damaged and needs replacement.
Identifying the diode's positive and negative ends
The cathode (negative end) of a diode is marked with a band or stripe, usually black or dark gray, around one end of the component. The anode (positive end) is the unmarked end. This marking is critical because testing a diode backwards will give you the wrong result — you need to touch the red probe to the anode and the black probe to the cathode for an accurate reading.
On some surface-mount diodes or components where the stripe is hard to see, you may need to look at the component's datasheet or use a magnifying glass. If you cannot find the marking, test the diode both ways: one direction should show a voltage drop, and the other should show no reading. Whichever direction gives you the voltage drop tells you which end is the anode.
What the multimeter display tells you
A healthy diode shows a voltage drop between 0.4 and 0.7 volts when forward-biased (red probe on anode, black probe on cathode). Silicon diodes typically read around 0.6 volts, while germanium diodes read closer to 0.3 volts. The exact number depends on the type of diode and the multimeter's internal test current, so do not worry if your reading is slightly different from another person's — what matters is that it falls within the expected range.
When you reverse the probes (black on anode, red on cathode), a working diode should show no reading, "OL", or a very high resistance. This reverse-bias state is the diode doing its job: blocking current from flowing backwards. If your diode shows a voltage drop in both directions, it is shorted and needs to be replaced. If it shows no reading in either direction, it is open (broken) and also needs replacement.
Testing diodes that are still soldered to a circuit board
Testing a diode while it is soldered to a board is possible but less reliable than removing it first, because other components in the circuit can interfere with the reading. If you must test in place, desolder one end of the diode or at least lift one leg slightly so it is not in electrical contact with the rest of the circuit. This breaks the path for current to flow through other components and gives you a clearer reading.
If you cannot desolder the diode, test it anyway and note that the reading may be lower or less clear than it would be on a removed component. A forward-bias reading of 0.4 to 0.7 volts is still a good sign even in circuit. A reading of zero or "OL" in both directions while the diode is in circuit is less conclusive — it might mean the diode is bad, or it might mean other components are blocking the test current.
Common reasons a diode test fails
A diode shows no reading in either direction when it is open — meaning the internal junction has broken and no current can flow at all. This usually happens from age, heat stress, or physical damage. A diode that shows the same voltage reading in both directions is shorted, meaning the internal barrier has failed and the diode conducts in both directions like a plain wire. Both conditions mean the diode cannot do its job and must be replaced.
Sometimes a diode appears to fail the test but is actually fine, and the problem is your multimeter. If your multimeter's battery is very low, the diode test mode may not send enough current to get an accurate reading. Replace the battery and try again. If you are testing a very large power diode, some multimeters do not send enough current to get a clear reading — in that case, you may need a different testing method or a different meter.
Diode testing on different multimeter types
Digital multimeters (the most common type) have a dedicated diode mode button or dial setting. Analog multimeters (with a needle) do not have a diode mode and should not be used for diode testing — the resistance setting on an analog meter can damage some diodes because it sends too much current. If you only have an analog multimeter, use the resistance setting on the lowest ohms range and treat any reading as inconclusive; a good diode may show low resistance in one direction and high resistance in the other, but this is not as reliable as a digital diode test.
Auto-ranging digital multimeters automatically select the right scale for the reading, which makes diode testing easier. Manual-range meters require you to select the diode mode, but the process is the same. Some multimeters display the voltage drop as a number, while others show "OL" or a bar graph — check your meter's manual if you are unsure how to read the result.
Frequently Asked Questions
Can I test a diode without removing it from the circuit?
Yes, but the reading may be affected by other components in the circuit. For the most accurate result, desolder one end or lift one leg so the diode is isolated. If you cannot do that, a forward-bias reading of 0.4 to 0.7 volts is still a good sign the diode is working, but a zero reading does not necessarily mean the diode is bad.
What voltage should a good diode show?
A silicon diode typically shows 0.5 to 0.7 volts in forward bias. A germanium diode shows 0.2 to 0.3 volts. The exact number depends on your multimeter and the diode type, so check the component's datasheet if you need a precise specification. The key is that it shows some voltage drop in one direction and no reading in the other.
Why does my diode show a reading in both directions?
A diode that conducts in both directions is shorted and needs to be replaced. This usually happens from heat damage, age, or a manufacturing defect. A shorted diode cannot block reverse current, so it will not work correctly in any circuit.
Can I use the resistance setting instead of diode mode?
The resistance setting is less reliable for diodes because it does not send the right amount of current. Use the diode mode if your multimeter has one. If you only have resistance mode, a low reading in one direction and a high reading in the other suggests the diode is working, but this is not as conclusive as a proper diode test.
What if my multimeter does not have a diode mode?
Older or very basic multimeters may not have a diode mode. You can use the lowest ohms resistance setting, but the reading will be less reliable. A working diode shows low resistance in one direction and very high resistance in the other. If you test frequently, consider upgrading to a multimeter with a dedicated diode mode.