What a diode test tells you
A diode is a one-way valve for electricity — it lets current flow in one direction and blocks it in the other. When you test a diode with a multimeter, you are checking whether it still does that job. A working diode shows low resistance in one direction (the forward direction) and very high resistance in the other (the reverse direction). If both directions show the same resistance, or if both show infinite resistance, the diode is dead.
Most multimeters have a dedicated diode test mode, marked with a diode symbol that looks like a triangle with a line across it. This mode 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. That number tells you the diode is conducting. If you see "1" or "OL" (overload), the diode is blocking current, which is what you want to see when you reverse the leads.
You test a diode in-circuit (while it is soldered to a board) only if you are confident no other components will interfere with the reading. Out-of-circuit testing — removing the diode first — is more reliable and is the standard approach when you are troubleshooting.
Key Takeaways
- Set your multimeter to diode mode (the symbol looks like a triangle with a line) and connect the red probe to the positive side of the diode and the black probe to the negative side.
- A working diode shows a voltage drop between 0.4 and 0.7 volts in the forward direction; reverse the probes and you should see "1" or "OL" (infinite resistance).
- If both directions show the same reading, or if both show infinite resistance, the diode has failed and needs replacement.
- Removing the diode from the circuit before testing gives you a cleaner result than testing it while soldered in place.
Setting up your multimeter for diode testing
Turn your multimeter on and locate the diode symbol on the dial or mode selector. On analog multimeters, this is usually on the resistance scale. On digital multimeters, it is often a separate button or a position on the rotary dial — check your manual if you cannot find it. The symbol resembles a triangle pointing right with a vertical line on the right side.
Once you have selected diode mode, you do not need to worry about selecting a range — the multimeter handles that automatically. Plug the red probe into the positive (VΩmA) jack and the black probe into the common (COM) jack. These are the standard jacks on any multimeter. If your probes are not already connected, attach them now — the red probe to the red jack and the black probe to the black jack.
Testing the diode in the forward direction
The forward direction is the direction the diode is designed to conduct. On a physical diode, a band or stripe around one end marks the cathode (negative side). The other end, unmarked, is the anode (positive side). Current flows from anode to cathode inside the diode.
Touch the red probe to the anode (the unmarked end) and the black probe to the cathode (the banded end). Look at the multimeter display. A working diode shows a voltage drop — typically 0.4 to 0.7 volts for a silicon diode, or 0.2 to 0.3 volts for a germanium diode. The exact number depends on the diode type, but any reading in that range means the diode is conducting and the junction is good.
If the display shows "1" or "OL" (overload), the diode is not conducting in the forward direction, which means it has failed. If the display shows "0" or a very low number like 0.01 volts, the diode may be shorted internally — a different kind of failure, but still a failure.
Testing the diode in the reverse direction
Now swap the probes. Touch the black probe to the anode and the red probe to the cathode. In this reverse direction, a working diode should block current almost completely. The display should show "1" or "OL" (infinite resistance), meaning no current is flowing.
If the reverse direction also shows a voltage drop (like 0.5 volts), the diode has failed. A diode that conducts in both directions is shorted and cannot do its job. If both directions show "1" or "OL", the diode is open (broken internally) and also cannot conduct in either direction — another failure mode.
The key result is a difference between the two directions: low voltage drop forward, infinite resistance reverse. That pattern means the diode is working.
Reading the results correctly
A healthy diode shows two opposite behaviors. Forward direction: 0.4 to 0.7 volts (or 0.2 to 0.3 for germanium). Reverse direction: "1" or "OL". If you see this pattern, the diode is good and you can put it back in the circuit or move on to testing other components.
If the forward direction shows "1" or "OL", the diode is open and will not conduct. If the reverse direction shows a voltage drop instead of "OL", the diode is shorted and will conduct in both directions. Either way, the diode has failed. If both directions show the same reading, something is wrong with the diode or your test setup — check that you have selected diode mode and that your probes are making good contact.
Some multimeters display the voltage drop as a negative number in reverse mode. That is normal — it is just the meter's way of showing polarity. What matters is whether you see a voltage drop (forward) or infinite resistance (reverse).
Common mistakes that give false results
The most common error is testing a diode while it is still soldered to the circuit board. Other components connected to the diode can conduct current and interfere with your reading. If you get confusing results, desolder the diode and test it alone. Use a soldering iron or desoldering pump to remove it cleanly, then test it on the bench.
Another mistake is reversing the diode symbol in your mind. The band marks the cathode (negative), not the anode. If you touch the probes to the wrong ends, you will get the opposite results — "OL" forward and a voltage drop reverse — which looks like a failed diode when the diode is actually fine. Double-check the band location before you conclude the diode is bad.
A third error is confusing diode mode with resistance mode. In resistance mode, a working diode will show very high resistance in both directions, which looks like a failure. Make sure the dial or button is set to the diode symbol, not the ohm symbol (Ω).
When to test a diode and when to replace it
Test a diode when a circuit is not working and you suspect a power supply, rectifier, or protection diode has failed. Diodes are cheap — often under a dollar — so if testing confirms it is bad, replacement is usually faster than troubleshooting further. If you are not sure whether the diode is the problem, testing takes two minutes and rules it out.
Some circuits have multiple diodes. If one has failed, test the others too. A failed diode often means something else in the circuit was drawing too much current or the diode was undersized for the job. Replacing only the diode without understanding why it failed can lead to the same failure again.
If you do not have a multimeter with diode mode, you can use the resistance setting as a rough check — a working diode shows high resistance in one direction and very high resistance in the other — but the dedicated diode mode is more reliable because it applies a specific test current designed for diodes.
Frequently Asked Questions
What is the difference between forward and reverse bias?
Forward bias is when you explore voltage to push current through the diode in its intended direction (anode positive, cathode negative). Reverse bias is the opposite polarity. A working diode conducts easily under forward bias and blocks current under reverse bias. Testing checks both conditions to confirm the diode works both ways it should.
Can I test a diode while it is soldered to the board?
You can try, but other components in the circuit may conduct current and give you a false reading. Desoldering the diode and testing it alone is more reliable. If you must test in-circuit, disconnect one end of the diode from the board first, or test only if you are confident no parallel paths exist.
What does "OL" mean on my multimeter?
OL stands for overload or open line. It means the multimeter is reading infinite resistance — no current is flowing. In diode mode, "OL" in the reverse direction is the correct result for a working diode. In the forward direction, "OL" means the diode is not conducting and has likely failed.
Why does my diode show different voltage drops each time I test it?
Small variations (0.1 to 0.2 volts) are normal and depend on the diode type, temperature, and the multimeter's test current. If the voltage drop is consistently in the 0.4 to 0.7 volt range for silicon, the diode is fine. If it changes dramatically or drops to zero, the diode may be failing.
How do I know which end of the diode is the cathode?
The cathode is marked with a band or stripe around one end of the diode body. The other end, unmarked, is the anode. On a diode symbol, the line points toward the cathode. If you cannot see the band clearly, use a magnifying glass or test both ways — one direction will show a voltage drop and the other will show "OL".