Last Updated: March 23, 2026
What You’ll Need
- Digital multimeter with diode test mode (diode symbol: ->|-)
- Test probes
- Diode to test (removed from circuit or with one lead disconnected)
- Non-contact voltage tester (to confirm the circuit is de-energized)
Safety Precautions
- Disconnect the diode from live circuits: Never test a diode on an energized circuit. The test voltage from the meter is very small — live circuit voltage will overwhelm it and damage the meter
- Discharge capacitors before testing: Circuits with large capacitors (power supply filter caps, motor start caps) may still have stored voltage after power is removed. Discharge safely before touching diode leads
- Disconnect at least one diode lead for accurate results: In-circuit testing often gives misleading results because other components create parallel paths. Remove the diode or disconnect one lead
Understanding Diode Test Mode
Diode test mode on a multimeter applies a small forward voltage (typically 2V) and measures the voltage drop across the diode. Unlike resistance mode, it measures millivolts (mV) of forward voltage drop, not ohms. The results tell you:
- Forward voltage drop (0.4–0.7V for silicon): The diode is functioning normally
- OL (overload) in forward direction: The diode is open (failed open circuit)
- Near 0V in both directions: The diode is shorted
- OL in both directions: Open diode or testing in wrong mode
Step-by-Step: How to Test a Diode
Remove or isolate the diode
Remove the diode from the circuit, or desolder/disconnect one lead so the diode is isolated from parallel paths. Identify the cathode (marked with a stripe or band on the body) and anode (the other terminal).
Set the multimeter to diode test mode
Rotate the dial to the diode symbol (→|-). Some meters share this position with continuity mode — look for the diode arrow symbol. Insert black probe in COM, red probe in VΩ.
Test forward bias (red to anode, black to cathode)
Touch the red probe to the anode (+) terminal and the black probe to the cathode (–) terminal. This is forward bias — current wants to flow through.
Good silicon diode: Reads 0.450–0.750V (typically around 0.5–0.65V for a standard rectifier diode)
Good Schottky diode: Reads 0.150–0.450V (lower forward voltage than silicon)
Good LED: Reads 1.8–3.5V depending on color (red ~2V, blue/white ~3–3.5V)
Shorted diode: Reads near 0.000V (no forward voltage drop — shorted)
Open diode: Reads OL (no forward current — diode is open/failed)
Test reverse bias (red to cathode, black to anode)
Swap the probes — red to cathode, black to anode. This is reverse bias — current is blocked.
Good diode: Reads OL (infinite resistance — blocking as expected)
Shorted diode: Reads the same forward voltage value even in reverse direction
Leaky diode: Reads a low voltage value in reverse — means the diode is failing and not blocking correctly
Interpret combined results
Forward Reading Reverse Reading Diode Status 0.5–0.7V OL ✅ Good silicon diode 0.15–0.45V OL ✅ Good Schottky diode 1.8–3.5V OL ✅ Good LED 0.000V or near 0 0.000V or near 0 ❌ Shorted diode OL OL ❌ Open diode Any value Low voltage (not OL) ❌ Leaky/failing diode
Practical Applications
Testing Rectifier Diodes in a Bridge Rectifier
Bridge rectifiers (used in power supplies, battery chargers, alternators) contain 4 diodes. Test each diode individually: good = forward voltage in one direction, OL in the other. A shorted diode in a bridge rectifier causes the output fuse to blow. To test an alternator, see our guide on how to test a car alternator with a multimeter.
Testing an LED
In diode test mode, a good LED actually lights up dimly during the forward-bias test — the test current (typically 1–2mA) is enough to emit light. This confirms the LED is functional. A dead LED reads OL in forward direction.
Testing Zener Diodes
Standard diode test mode checks forward conduction normally (0.5–0.7V forward for most Zeners). Zener breakdown (reverse conduction) requires reverse voltage above the Zener voltage — which the meter’s low test voltage can’t reach. For Zener testing, test forward voltage only; the reverse breakdown requires a variable DC supply above the Zener voltage.
Testing Protection Diodes on Circuit Boards
ESD protection diodes, flyback diodes on relay coils, and reverse-polarity protection diodes all test with the same forward/reverse bias method. These small signal diodes typically have slightly different forward voltages (0.3–0.6V) than power rectifiers.
Identifying Diode Polarity
Before testing, you need to know which end is the anode (+) and cathode (–):
- Through-hole diodes: A stripe or band at one end = cathode. The unmarked end = anode.
- LEDs: The longer lead = anode. Shorter lead = cathode. (On used LEDs, check the flat edge of the lens body — flat side = cathode.)
- Surface-mount diodes: Mark varies by manufacturer — check the datasheet. Typically a line or dot at the cathode end.
- Unknown orientation: Touch red probe to one end and black to the other. If you get a forward voltage reading (0.4–0.7V), red is at the anode. If OL, reverse the probes.
Pro Tips
- LED color indicates testing voltage: Red LEDs have the lowest forward voltage (~1.8–2.2V). Blue and white LEDs have the highest (~3.0–3.5V). If your meter’s test voltage is only 2V, it can’t fully forward-bias high-voltage LEDs and they may read OL even when good
- Some meters have separate LED test ranges: Higher-end meters can test LEDs with higher forward voltages. Check your meter’s spec for maximum diode test voltage
- Use resistance mode as backup: If your meter doesn’t have diode test mode, use the lowest resistance range. A good diode reads low resistance in forward bias and very high (or OL) in reverse. This doesn’t give you a precise voltage drop but confirms basic function
- Test both diodes in a dual-diode package: Many power transistors include built-in diodes. Test both the main diode and any internal body diode separately
Frequently Asked Questions
What should a good diode read on a multimeter?
In diode test mode: forward bias (red to anode) should read 0.4–0.7V for silicon diodes, 0.15–0.45V for Schottky diodes, and 1.8–3.5V for LEDs. Reverse bias (red to cathode) should read OL for all types.
Why does my diode read OL in both directions?
The diode is open-circuit — the internal semiconductor junction has failed. The diode can no longer conduct in either direction. Replace it.
Why does my diode read the same in both directions?
The diode is shorted — it conducts equally in both directions like a wire. Common causes: overvoltage, reverse voltage spike above the reverse breakdown voltage, or overcurrent thermal failure.
Can I test a diode with it still in the circuit?
Sometimes, if one end is isolated. But many in-circuit tests give misleading results because of parallel paths through other components. For reliable results, remove the diode or disconnect one lead.
What’s the difference between the diode test mode and resistance mode for testing diodes?
Diode test mode applies a fixed test voltage and measures the forward voltage drop — giving you the actual forward voltage value. Resistance mode measures a rough resistance value but may not accurately represent the diode’s real behavior. Diode test mode is specifically designed for this test and gives more meaningful results.
Conclusion
Testing diodes is one of the faster multimeter skills to learn. Set diode mode, check forward voltage in one direction (should be 0.4–0.7V for silicon), and OL in the other direction. Any deviation from this pattern tells you the diode is shorted, open, or leaky. This test applies to everything from cheap rectifiers to LEDs, bridge rectifiers, and alternator diodes — always the same two-step forward/reverse process.
