Testing the Brake Light System Fuse
If none of your three brake lights is working, it's doubtful that all the individual bulbs have burned out. It's far more likely you have an electrical system problem. It may sound complicated and expensive, but don't worry. Odds are the electrical system trouble is nothing more than a failed brake light system fuse.
Each part of the car's electrical system corresponds to a fuse, which protects it from amperage overload; if one electrical component blows out, the rest of the car doesn't get fried. If the fuse fails, electricity can't reach the lights, which may be in fine working order otherwise.
Like all fuses, the brake light system fuse can be found in the power distribution center, which is under the dashboard or tucked away underneath the hood. Never heard of the power distribution center? It's just the technical name for a fuse box. Using your car's manual, locate the fuse that correlates to the brake lights.
You'll need to connect a test light for the next step (you can get one at any auto parts store). Turn your car's ignition to the "on" position, grab the test light and attach it to a ground source, like the dash or the body of the car and gently press the tip of the tester to each of the fuse's two ends. Now, press down lightly on the gas pedal. Does the test light illuminate? If so, the fuse is functional, and the problem most likely is a used-up brake light bulb. If, however, the test light illuminates when the connection is only pressed against one side of the fuse, the fuse is faulty. If the test light fails to light at all, regardless of where it's connected to the fuse, you definitely need to replace the fuse. Make sure the replacement also has the same amp rating as the original fuse.
But wait, you're not done! Once you put in a new fuse, you'll need to test it again. If both lights trigger, the fuse is working and the repair is complete. Be aware that a blown fuse is often an indicator of electrical faults higher up in the system. In such a case, the new fuse will burn out as well and you should consult an electrical technician to pinpoint the exact location of the fault.
If the new fuse doesn't work while you have a foot on the pedal, the circuit itself is shorted out. That's a more complicated and expensive fix, which will need to be handled by a professional mechanic.
If you're lucky, maybe you just need a new light bulb. We'll show you how.