Symptom:

Brake lights do not go on when you squeeze the brakes

Solution:

There are two types of brake switches. The newer ELFs (as of at least August 2015), have a mechanical plunger switch built into the right hand brake lever. The older ELFs have a reed switch / magnet combination mounted next to the front left brake assembly on the tire.

Check to see if you have a wire (not just the two brake cables) coming out of your right hand brake lever on the handle bar. The picture to the left shows this wire. If you have a wire then then you have the newer style.

Older ELF reed switch plus magnet

As the brakes wear the caliper moves more and more to pinch on the disk. More movement requires more pulling on the cable. When you pull the cable the magnet hits the sensor, after a while the sensor pushes the magnet up the cable a little more. You can adjust the magnet on the cable to make the lights work again, but a better solution is to adjust the brakes so that the cable doesn't have to pull as far.

If you have the older reed switch and magnet system and the brake lights do not come on, check to see if the magnet has come off. A standard spoke-magnet used on many bicycle speed sensors can be used to replace a missing magnet.

Newer ELF with plunger built into break lever

Check to make sure that the brake cables are not adjusted too tight. If you can’t pull the brake lever to engage the self locking feature it is adjusted too tight. This will prevent the mechanical plunger switch from turning on your brake lights. I also have had intermittent failures with the plunger switch not engaging properly. Gently, try pulling and wiggling the wire from the brake handle with the brake lever pulled in to see if the lights turn on.