How to Test a Transistor With a Multimeter

First, which transistor?

The test method varies by type. A BJT has three legs (base, collector, emitter) and behaves like two diode junctions inside; that's why it's easily checked with the diode test. A MOSFET, being voltage-controlled, is tested a bit differently. You can find the operating difference between the two in the MOSFET vs BJT article.

BJT test: diode mode

Set the multimeter to diode-test mode. A BJT contains two PN junctions: base-emitter and base-collector:

For NPN: place the red (+) probe on the base. Touch the black probe in turn to the collector and the emitter; you should read a drop of about 0.5–0.7 V on both (conduction).
Reverse the probes (red to C/E): there should be no reading in this direction (isolation).
There should be no conduction between collector and emitter in either direction.

For a PNP the directions are reversed: the black (−) probe goes on the base. If you see the expected values, the transistor is most likely good.

Signs of a faulty transistor

If you see conduction (near 0 V) in both directions between two legs, that junction has shorted. If there's no conduction in any direction, the junction is open-circuit. Both indicate the transistor is faulty. A short between collector and emitter in particular is the most typical sign of a transistor burned out by excess current.

MOSFET test

A rough check can be done on a MOSFET with diode mode too: the body diode between source and drain should conduct in one direction. For a more realistic test, you need to "charge and discharge" the gate by hand: briefly touching a positive voltage to the gate should turn the MOSFET on (the drain-source resistance should drop), and discharging the gate should return it to isolation. Because the gate is sensitive, be careful of static (ESD) during the test.

Determining the pinout

If you don't know which leg is base/collector/emitter or gate/drain/source, look up the part's MPN in its datasheet; the pinout can differ by manufacturer. A wrong leg assumption leads you to think a good transistor is "faulty." You can review datasheet reading in the 7 critical datasheet parameters article.

Why does a transistor fail?

Understanding why a transistor failed is as important as testing it; otherwise the new one shares the same fate. The most common causes: drawing more current than allowed (insufficient current limiting), excess heat (missing heatsink), a voltage spike from an inductive load (no flyback diode fitted) and, in MOSFETs, static exceeding the gate. Fixing this root cause in the circuit before installing the new transistor provides a lasting solution.

Common mistakes

Assuming the leg order: testing without verifying the pinout from the datasheet gives the wrong result.
Measuring in-circuit: parallel components corrupt the reading; disconnect the transistor at least partially from the circuit.
Confusing NPN/PNP: the probe direction depends on the type; in the reverse direction everything looks "faulty."
Ignoring the MOSFET gate: when the gate stays charged, the MOSFET can appear open; discharge it first.

Frequently asked questions

My multimeter has no diode mode, what do I do?

You can check with similar logic using resistance mode; but diode mode is much clearer because it shows the junction voltage.

Do transistor testers (hFE) work?

Some multimeters have an hFE socket and measure gain; it's practical but the leg order must be inserted correctly. Dedicated transistor testers also find the pinout automatically.

It tests good but the circuit doesn't work?

The drive circuit, base resistor or supply could be faulty; check the components around the transistor too.

Conclusion

Testing a transistor is a simple routine that first determines its type and pinout and then checks the junctions with diode mode. This lets you quickly eliminate a suspect part and avoid an unnecessary swap. When looking for a new one or an equivalent, you can use search and the comparison tool to find the right part.