Q: My fuse for the brake lights keeps popping. I was made aware of the problem after almost being rear-ended, then figured out the stop lights weren’t working. I’ve had to replace the fuse several times over the past month or so and am getting frustrated having to constantly check the lights or worry when they’ll stop again. Any ideas of what I could check before bringing it in?

A: This is a creepy situation that needs to be resolved right away. I’d start by looking for details regarding this fuse in either the vehicle owner’s manual or a professional-grade automotive service database such as On-Demand-5 or Alldata, which may be available at your public library. Your first objective is to determine if other circuits such as the horn are protected by the same fuse. This is commonly done, I believe, in a safety-related circuit, to make a blown fuse more noticeable. Knowing what is protected helps you create a shopping list for components and wiring to be checked, and what isn’t of concern. Next, purchase a box or two of five fuses of the correct amperage — you’ll pop a few during our test procedures. Never install a larger fuse.

Let’s assume your horn and brake lights share the same fuse, which places both circuits on our list. Have you used the horn during the period of time when the fuse has popped? Try renewing the fuse, then honk the horn for perhaps five or 10 seconds and see what happens. If the fuse blows, locate the horn and unplug it; there might be more than one. Renew the fuse and repeat the test. If the fault goes away, plug the horn back in, and see if the fuse again fails when the horn sounds. This would pin the fault to the horn unit. If the fuse pops with the horn unplugged, the fault is likely in the wire between the horn relay and horn. If there are two horns, try plugging them in one at a time. A visual inspection of the wiring would be your best chance at finding a wire injury; it’s likely to be chafing against a grounded, sharp metal object.

If the horn works well with no fuse issues, try the same test with the brake lights, this time pressing the pedal for perhaps one minute continually. If the fuse pops after pressing on the brake pedal, the fault is between the brake pedal switch and the brake lights. Following this circuit will likely require detailed circuit information found in a pro database. The objective will be to locate all connectors between the two places, unplug them and renew the fuse. Then with the brake pedal continually pressed (use a stick or similar) plug them back in, one at a time from back of car to front. When the fuse pops, you’ve located the fault area. Short circuits often happen in or near lamp sockets or near moving or hot machinery; they may surface via vibration. Other causes of a fuse blowing can be too many or too large accessories or faulty components.

Intermittent shorts can be tough to find. Purchase an inexpensive buzzer, such as Radio Shack’s model 273-055 for about $4, and press its two slender wires into the two sides of the fuse cavity prior to gently installing a new fuse. When the fuse pops, the buzzer will sound. This way you’ll know what the fault conditions were — did I honk or brake? — and you’ll also have immediate warning the brake lights are no longer functional. If none of this helps, seek professional help immediately.