A P-channel MOSFET, or metal oxide semiconductor field-effect transistor, is a voltage-sensitive device used in electronic switching and amplifying circuits. If you have a piece of electronic equipment with problems and suspect a bad MOSFET, you can easily check it with a digital multimeter, or DMM, and a 9-volt battery. This test works whether the MOSFET is in or out of circuit. You'll need to know if the device is an enhancement or depletion mode MOSFET, as this affects the test.
Turn the device off and unplug it from the AC outlet. Allow a few minutes for the capacitors to discharge before testing the MOSFET.
Examine the MOSFET's data sheet and determine which of its leads are the source, gate and drain. Also determine if the device is an enhancement or depletion mode MOSFET.
Turn the DMM on and set its function knob to "diode check." Touch the positive meter probe to the MOSFET's source lead and the negative probe to the gate lead. Both enhancement and depletion-mode devices should read "overflow" or very high resistance.
Clip a test lead between the device's source and the gate leads. Touch the positive meter probe to the source and the negative to the drain. An enhancement-mode device will read "overflow" on the meter. A depletion-mode MOSFET will read a very low resistance, such as 1 ohm. Remove the test lead from the device.
Snap a fresh 9-volt battery onto the battery clip. Connect the positive battery clip wire to the MOSFET's source with one test lead. Connect the negative battery clip wire to the gate lead with another test lead. Touch the positive meter probe to the source and the negative to the drain. An enhancement-mode MOSFET will read low ohms resistance. A depletion-mode transistor will read "overflow." If the device fails any of these checks, replace it.
You can use the above steps to check an N-channel MOSFET, providing you reverse the meter's polarity. For example, you would touch the negative probe to the source and the positive to the drain.
Since MOSFETs are voltage-sensitive devices, they can be damaged by static electricity. If you handle them out of a circuit, use a grounded wrist strap to safely discharge static from your hands.