To diagnose brake light repair problems, test the brake-light switch, test the fuse, and replace the bulb assembly. The specific steps for diagnosing brake light problems depend on the car type and model.
Brake lights that stay on are a common problem that is usually caused by a faulty brake-light switch. Begin by removing the analog switch and testing between the terminals for continuity. A vehicle that uses a digital sensor is more difficult to test and requires professional assistance. Replace the faulty brake-light switch to fix the problem.
Another common problem with brake lights is that they do not come on. First, check that the third brake light is working. If not, inspect the vehicle for an unplugged harness, and gently push it back in place. Next, test the fuse and brake-light switch for continuity, and replace the faulty part to fix the problem.
Another problem with brake lights is that the lower lights work but the third brake light does not. Begin by switching on the emergency flashers to see if the brake light works. If it does, the fuse may be faulty. Test the fuse, and replace it if necessary. If the third brake light does not work with the emergency flashers on, the problem is probably bad bulbs. Bulbs usually need to be replaced as an assembly.