Diode Test Mode on a Multimeter: How to Read It

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The person typing diode test mode on multimeter explained wants to know what the diode setting does and how to read the number it shows. Diode test mode applies a small voltage and current to a junction and displays the voltage that remains across it when it conducts. A good silicon diode shows a forward drop of roughly half a volt to 0.8 volts, and it shows an open-loop reading when reversed. This guide walks through the procedure, the forward and reverse readings, and the fault signs in the two sources we read, and it notes where those sources give different values for a shorted diode.

Quick answer

• Turn off the circuit power and remove the diode from the circuit before testing.
• Set the meter to diode mode. Put the red lead on the anode and the black lead on the cathode, then read the forward drop.
• A good silicon diode reads about 0.5 to 0.8 V forward and OL in reverse.
• A diode that reads OL in both directions is open. A diode that reads a very low voltage in both directions is shorted.

How we chose

We chose two sources that describe the procedure directly. The first is a Fluke learning article on how to test diodes with a digital multimeter. The second is the Starting Electronics guide to a diode test using a multimeter, which we opened at its current address after a redirect from the older URL. We used the procedure steps, the typical readings and the fault patterns. We did not rely on search snippets, product pages or star ratings, and no product is recommended in this guide.

Preparing the test

The Fluke article says to turn off all circuit power and to discharge any charged capacitors, so that no voltage is present across the diode. It also says that you may need to remove one end of the diode from the circuit before you measure, because other parts in the circuit can give a reading that does not come from the diode alone. The Starting Electronics guide gives the same instruction: turn off the power and remove the diode from the circuit.

When the work involves mains or a live appliance, de-energize the equipment, lock out the supply according to your site procedure, and confirm it is dead before you connect the leads. A diode check is a low-voltage test, but the circuit it sits in may not be.

Setting the meter and connecting the leads

Set the dial to diode test. On some meters, the diode function shares a dial position with another function, so check the symbol on the meter before you start. The Fluke article describes the diode as forward-biased when the red lead, which is positive, sits on the anode and the black lead sits on the cathode. The cathode is usually marked by a band on the body of the diode.

Measure in that orientation first, then reverse the leads and measure again. The two readings tell you more than either reading alone.

Forward reading: what a good diode shows

For a good silicon diode, forward-biased, the Fluke article gives a voltage drop of about 0.5 to 0.8 volts. The Starting Electronics guide gives a typical range of about 0.6 to 0.7 volts, and it says a silicon diode often reads a bit lower than that. It gives 0.596 volts as an example of a good diode. The two sources agree on the general range, although the Starting Electronics guide describes the middle of the range more narrowly.

LEDs read higher. The Starting Electronics guide gives a typical range of about 1.8 to 3.3 volts for LEDs, depending on the colour. That range is a useful check when you test an LED rather than a rectifier diode.

Reverse reading: what a good diode shows

Reverse the leads. A good diode acts as an open switch in reverse bias, and both sources say it shows OL, which means open loop. The Starting Electronics guide notes that some meters show 1 instead of OL. If the reverse reading shows a voltage drop rather than OL, the diode is not blocking as it should.

Reading a bad diode

An open diode, meaning a diode that does not conduct, shows OL in both directions. The Fluke article says a good diode reads OL reversed, and that an open diode reads OL in both directions, so no current flows either way. The Starting Electronics guide makes the same point about an open-circuit diode.

A shorted diode, meaning a diode that conducts in both directions, shows a low voltage drop in both directions. The Fluke article gives a shorted reading of about 0.4 volts in both directions. The Starting Electronics guide gives a shorted reading of close to 0 volts in both directions. The two sources do not agree on the exact value. Both agree that the reading is the same in both directions, which is the fault signature. Treat any diode that reads the same both ways as failed.

Why the numbers differ between meters

The two sources give different values for the shorted case, and the forward reading depends on the meter’s test current. Compare any reading with a known good diode of the same type on the same meter before you decide what the number means.

Checking in circuit

The Fluke article says you may need to remove one end of the diode from the circuit. If a reading looks wrong while the diode is still connected, lift one end and measure again before you decide the diode has failed.

For related reading on this site, see testing a capacitor with a digital multimeter and plug testers.

What we could not verify

• The two sources give different values for a shorted diode, about 0.4 volts in the Fluke article and close to 0 volts in the Starting Electronics guide. We did not resolve which value a particular meter will show, so the guide presents both.
• Neither source gives the test current of a specific meter model. The forward reading depends on that current, so check the meter’s specification.
• Our sources do not cover power diodes, Zener diodes or high-current rectifiers in detail. The readings here describe the general diode check, not a rating test.
• Neither source gives the reading for a specific LED colour beyond the general range.

FAQ

Should a diode read OL in both directions?

No. A good diode reads a forward drop one way and OL the other. OL in both directions points to an open diode.

Why does my diode read zero?

A reading near zero in both directions points to a shorted diode, according to both sources. Confirm by lifting one end of the diode from the circuit before you decide.

Can I test a diode while it is still in the circuit?

You can take a reading, but parallel components can change it. Lift one leg before you decide the diode has failed.

Why does an LED read higher than a rectifier diode?

An LED has a higher forward voltage than a silicon rectifier. The Starting Electronics guide gives roughly 1.8 to 3.3 volts for LEDs, depending on colour.

Bottom line

A good silicon diode reads a forward drop of about 0.5 to 0.8 volts one way and OL the other. A reading that is the same in both directions means the diode has failed.

Sources

• Fluke, How to Test Diodes with a Digital Multimeter: https://www.fluke.com/en-us/learn/blog/digital-multimeters/how-to-test-diodes (opened 2026-10-30)
• Starting Electronics, Diode Test Using a Multimeter: https://startingelectronics.org/diode-test-using-multimeter/ (opened 2026-10-30)