What a diode test actually 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 works that way. A working diode will show low resistance in one direction and very high resistance (or no reading) in the other. A dead diode shows high resistance both ways, or low resistance both ways, which means it has failed.
Most multimeters have a dedicated diode test mode, marked with a diode symbol that looks like a triangle with a line next to it. This mode sends a small test current through the diode and measures what comes back. It is faster and more reliable than using the resistance setting, because the resistance setting does not send enough current to properly test a diode.
Key Takeaways
- Set your multimeter to diode test mode (the symbol looks like a triangle with a line) rather than using the resistance setting.
- Touch the red probe to the positive end of the diode and the black probe to the negative end — this direction should show a reading between 0.4 and 0.7 volts.
- Reverse the probes — the reading should jump to infinity or show "OL" (overload), meaning the diode is blocking current in that direction.
- If both directions show a reading, or both show infinity, the diode is dead and needs to be replaced.
- Remove the diode from the circuit board before testing, because other components can interfere with the reading.
Finding the diode test mode on your multimeter
Look at the dial or button panel on your multimeter. You should see a symbol that looks like a small triangle pointing right with a vertical line to its right — that is the diode symbol. On analog multimeters (the ones with a needle), this is usually on the resistance section of the dial. On digital multimeters, it is often a separate button or a position on the dial between resistance and voltage settings.
If you cannot find a diode symbol, your multimeter may not have a dedicated diode test mode. In that case, you can use the resistance (ohms) setting, but the reading will be less clear. Set it to the lowest ohms range available, usually marked as 2Ω or similar.
The correct probe placement and what the reading means
A diode has two ends: the positive end (called the anode) and the negative end (called the cathode). The cathode is usually marked with a band or stripe around the diode body. Touch the red probe to the positive end and the black probe to the negative end. In this direction, a working diode will show a reading between 0.4 and 0.7 volts on a digital multimeter, or a small deflection on an analog needle.
Now reverse the probes — red on the negative end, black on the positive end. A working diode should show "OL" (overload) or infinity on a digital display, or no needle movement on an analog meter. This means the diode is blocking current in the reverse direction, which is exactly what it should do.
The actual voltage reading in the forward direction varies slightly depending on the type of diode. Silicon diodes typically show 0.5 to 0.7 volts. Germanium diodes show 0.2 to 0.3 volts. If your reading is much higher or much lower than this range, the diode may be failing.
Why you need to remove the diode from the circuit first
If the diode is still soldered to a circuit board, other components connected to it can interfere with your test. Resistors, capacitors, and other diodes in parallel or series with the one you are testing will give you a false reading. The multimeter cannot tell the difference between the diode you want to test and the rest of the circuit.
Desolder the diode carefully using a soldering iron or desoldering pump, or clip one leg of the diode to break the connection. Once it is isolated, your test will be accurate. If you cannot remove it safely, you can still get useful information by testing it in the circuit — a reading of infinity in both directions usually means the diode is definitely dead, even if a forward reading could be misleading.
What different readings actually mean
A normal forward reading (0.4 to 0.7 volts for silicon) with a reverse reading of infinity or OL means the diode is working correctly. You can put it back in the circuit.
A low reading in both directions (for example, 0.5 volts forward and 0.4 volts reverse) means the diode has shorted — the one-way valve is broken and current flows both ways. The diode is dead and must be replaced.
A high reading in both directions (infinity or OL both ways) means the diode has opened — the valve is stuck closed and no current flows either way. The diode is also dead and must be replaced.
A very high forward reading (above 1 volt, or no reading at all) combined with a normal reverse reading suggests the diode is failing but may still work in some circuits. This is a borderline case — if the circuit is not working, replace the diode.
Common mistakes that give false readings
Testing a diode while it is still soldered in the circuit is the most common mistake. The surrounding components will mask the true behavior of the diode. Always desolder or isolate it first if you have the skill to do so safely.
Touching the probes to the wrong ends of the diode will reverse your results — you will see infinity when you should see a voltage, and vice versa. If your reading seems backwards, check that the red probe is on the positive (unmarked) end and the black probe is on the negative (striped) end.
Forgetting to switch out of resistance mode is another common error. The resistance setting sends much less current through the diode, so the reading will be unclear or missing entirely. Always use the dedicated diode test mode if your multimeter has one.
Leaving the multimeter on diode test mode after you finish can drain the battery faster than other modes, since the diode test continuously sends current. Switch back to voltage or resistance mode when you are done testing.
When a diode test is not enough
A multimeter diode test tells you whether the diode conducts in one direction and blocks in the other. It does not tell you whether the diode can handle the current and voltage it will see in your actual circuit. A diode might pass the multimeter test but still fail when you power up the circuit, if the circuit demands more current than the diode is rated for.
If you replace a diode and the circuit still does not work, the problem may not be the diode itself. Check the components around it — burned resistors, cracked capacitors, or damaged traces on the circuit board can cause the same symptoms. A multimeter test is a good first step, but it is not a complete diagnosis.
Frequently Asked Questions
Can I test a diode while it is still in the circuit?
You can, but the reading will likely be wrong because other components interfere. If you see infinity in both directions, the diode is probably dead. If you see a reading in both directions, desolder it and test again to be sure.
What if my multimeter does not have a diode test mode?
Use the lowest ohms (resistance) range instead. A working diode will show low resistance in one direction and very high resistance in the other. The reading will be less precise than a dedicated diode test, but the pattern will be the same.
Why does the forward reading change depending on which end I touch first?
It should not change — the diode should always show low resistance when red is on the positive end and black is on the negative end. If the reading flips when you reverse the probes, you may have the ends backwards. Check the stripe on the diode body to confirm which end is negative.
Is a diode bad if it shows 0.9 volts in the forward direction?
Probably not, but it is on the high side for a silicon diode. Most silicon diodes read 0.5 to 0.7 volts. A reading above 0.8 volts suggests the diode may be aging or damaged. If the circuit is not working, replace it and see if that fixes the problem.
Can I test a light-emitting diode (LED) the same way?
Yes, an LED will show the same forward and reverse pattern as a regular diode. A working LED will show 1.5 to 2.5 volts in the forward direction (higher than a regular diode) and infinity in reverse. If the LED shows low resistance both ways, it is burned out.