How to Test a Diode with a Multimeter (Diode Mode Guide)

When you set the multimeter to diode mode (usually the diode symbol ▷|), it applies a ~2–3 V test voltage between the probes and shows the voltage dropped across the component in volts. It does not show resistance (ohms) — it shows the forward voltage.

When the red probe touches the anode and the black probe the cathode, the diode conducts and you read a voltage on the display. With the probes reversed, the diode does not conduct.

• Forward direction (red=anode, black=cathode): Typically 0.5–0.7 V on a silicon diode. Lower on Schottky diodes (~0.15–0.45 V).
• Reverse direction (probes reversed): The display shows OL (or 1, open circuit) — the diode doesn't conduct.

So a healthy diode: a meaningful voltage in one direction, OL in the other. This asymmetry is proof the diode works.

• Short: If you read ~0 V (or very low) in both directions, the diode is shorted — it now passes current both ways.
• Open: If you read OL in both directions, the diode is broken open — it conducts in neither direction.
• Leaky: If you read a low voltage instead of OL in reverse, the diode has weakened.

Short and open are the two classic forms of board-level failure; diode mode tells them apart in seconds.

Diode mode isn't only for a single diode — it's also used to read the health of a line against GND on the board. You measure in diode mode between a power line and ground:

• On healthy lines you usually see a meaningful value above zero (the natural behavior of the capacitors/semiconductors on that net).
• A very low reading near 0 V is the first sign that the line may be shorted to ground.

Important: the value read on the board belongs not to a single component but to the entire circuit tied to that point. So diode mode is a first-pass filter — a definitive diagnosis is completed by mapping the line and observing current draw.