When your favourite electronic device suddenly stops working, a blown fuse is usually the first suspect. The sole purpose of a fuse is to interrupt power to the device when it begins to draw excessive current. This protects the device from additional damage caused by excessive current and also protects the power source from a potentially dangerous overload caused by a short in the device. A digital multimeter is the ideal tool for testing for blown fuses.
Test the battery by turning on the digital multimeter and observing the digital display. If there is no display, or if the display shows characters with missing segments, replace the battery. Some digital multimeters will include a battery-test button, which activates a built-in battery tester.
Plug in the red and black leads if they are not permanently connected to the digital multimeter. The black lead plugs into the "Common" socket and red one into the "Volts" or "Ohms" socket. Touch the tips of the red and black leads together. The display should read all zeros, indicating no resistance. This is the same reading a new fuse would generate.
Measure the resistance of the fuse by placing one of the leads on each end of the fuse. Be careful to ensure that your fingers do not touch either lead; this could produce a false reading of resistance through your body. A good fuse will read 0.00, indicating that the fuse element is intact and not blown. Any other reading indicates a blown fuse.
A blown fuse does not indicate for certain that there is a problem with the fused device. Fuses, like other electronic parts, can fail due to age. Do a practice run by testing a fuse that is known to be new. If you elect to replace a defective fuse, only use fuses with the same voltage and current rating.
Digital multimeter design varies from model to model. If you are in doubt as to which settings to use or where to plug in the red and black leads, refer to the multimeter's manual. Make sure the power is removed from the device before removing the fuse. Never test a fuse while it is connected to the circuitry it is protecting.