Troubleshooting non-working brake lights involves a visual check of the emergency flashers, wire harness connections and bulbs, as well as testing the fuses, wiring and switches. The pieces of equipment necessary to perform the diagnostics include a fuse testing device and an ohmmeter.
The first step should be to turn on the vehicle's emergency flashers. If they work, it eliminates the bulbs, fuses or wiring as the source of the problem, and generally indicates an issue with a brake switch or relay. If the brake lamps are not illuminated, inspection of the bulbs is the next step.
All automobiles that do not use LED lighting have a 12-volt bulb in the brake light compartment. The bulbs are clear, with a connecting filament inside. If the filament is burnt or broken, the bulb needs replacing.
Fuse testing is accomplished by using a circuit testing device available at most auto parts stores. It generally consists of two probes with an indicator lamp. The fuses associated with the brake system are generally in the fuse box located in the passenger compartment. A diagram of the fuse locations and functions can assist in identifying the correct fuses to test. It can usually be found in either the fuse box lid or the vehicle owner's manual.
Checking the brake switches and relays can be more complex, depending on the vehicle's electrical system. The wiring system can be traced from the brake lights forward, with any harness plugs checked for continuity using an ohm meter. The brake switch is generally located under the brake pedal and can be tested with the ohm meter as well.