Blown fuse(s).If one particular accessory, set of lights, or convenience in your vehicle stops functioning altogether, a fuse has probably been triggered. Fuses are simple to diagnose, find, and replace yourself. Just look in your owner’s manual for the location of the fuse panel (your car might have more than one), and the location of the particular fuse for the failed accessory.
Dead battery, or bad battery terminals. Auto batteries can fail without warning, if they’ve been in service for two years or more or if they’re defective. If you have no juice, make sure your battery terminals are tightened and clean of corrosion. If they’re not, then that might be the cause. If they are snug and clean, use your voltmeter to check the battery by touching the meter’s probes to the battery’s plus and minus posts, scratching slightly to ensure a good connection. With the engine and ignition off, if the voltmeter reads between 10 and 12 volts, then the battery is good (9-9.5 is barely acceptable).
Alternator. A newer battery that won’t keep a charge, the dimming of headlights, or stalling at stoplights are all signs of an alternator that is about to fail completely. If you suspect that your charging system is giving out, test it for certain by touching the probes of your voltmeter to the plus and minus terminal clamps when the engine is running. If it registers between 12.8 and 14.7 volts, this is usually considered within the normal range and your charging system is fine. If the voltage is too low, try the voltage at the alternator output terminals, and if it measures as low, then your alternator likely needs replacement.
Fusible link. A fusible link is a higher-capacity connector that protects accessories wired directly from the battery, including the starter. Fusible links can either fail gradually or suddenly, depending on the cause. Sluggish electrical accessories, an unusual pause before the starter turns, and odd behavior of accessories plugged into the power socket are all symptoms of a failing fusible link. If the electrical system is completely dead, the battery cables are tight, and jump-starting does absolutely nothing, then the fusible link has likely failed or been triggered. For replacement, get a proper repair manual for your car or take it to your mechanic.
Faulty switch. Test the switch that has failed in all possible positions, to see if the accessory works at all, or to test for loose connections or shorts. Particular switches, such as those for headlights, sometimes have built-in circuit breakers. Consult with your owner’s manual or shop manual on how to reset the breaker.