A diode conducts current in one direction and blocks it in the other — and testing whether it still does that correctly is one of the fastest fault-finding checks you can run with a multimeter. Most digital multimeters have a dedicated diode test mode for exactly this purpose. This guide covers how to use it, what a good reading looks like, and how to catch the three common diode failure modes.
Using the Diode Test Function
Set the multimeter's rotary dial to the diode test symbol (a diode symbol with an arrow, often sharing a position with continuity test). In this mode, the meter applies a small test current through its probes and displays the forward voltage drop across whatever is connected — unlike resistance mode, which is not designed to characterise a semiconductor junction correctly.
- Ensure the circuit is fully powered off and, ideally, the diode is isolated from the circuit — a diode still wired into a circuit can be influenced by parallel paths and give a misleading reading.
- Connect the red probe to the diode's anode and the black probe to the cathode (the cathode is marked with a printed band on the diode body).
- Read the display: a healthy silicon diode shows a forward voltage drop, typically in the 0.5–0.8 V range.
- Reverse the probes. A healthy diode should now show "OL" (open loop/overrange) — no conduction in the reverse direction.
| Diode type | Typical forward voltage |
|---|---|
| Silicon rectifier/signal diode | 0.5 – 0.8 V |
| Germanium diode | 0.2 – 0.3 V |
| Schottky diode | 0.15 – 0.45 V |
| Standard LED | 1.8 – 3.3 V (colour-dependent) |
LEDs sometimes glow faintly in diode test mode
The Three Diode Failure Modes
- Shorted diode: Reads a low voltage drop (often close to 0 V) in both directions. The junction has broken down and the diode now conducts both ways — a shorted diode is a common cause of a blown fuse or dead-short symptom elsewhere in the circuit.
- Open diode: Reads "OL" in both directions. The junction is broken and the diode no longer conducts at all, even forward-biased.
- Leaky diode: Shows the correct forward reading, but the reverse direction shows some finite voltage/conduction instead of a clean "OL" — the diode still functions but with degraded reverse-blocking performance, which can cause intermittent or load-dependent circuit faults that are harder to diagnose than a clean short or open.
Testing a Diode In-Circuit
A diode tested while still connected in a circuit can show a false reading if other components — resistors, other diodes, transformer windings — provide a parallel current path around it. If an in-circuit reading looks ambiguous or doesn't match the expected values above, desolder at least one leg of the diode and retest in isolation before concluding it has failed.
CIE supplies digital multimeters with built-in diode and continuity test functions for bench and field fault-finding. Contact us to find the right model for your workshop.