What Is a Third Brake Light? | Safety Law & Why It Matters

The third brake light, officially the Center High-Mount Stop Lamp (CHMSL), is a mandatory safety light at the center of a vehicle’s rear that illuminates only during braking to reduce rear-end collisions.

One wrong tap on the brake pedal and the car behind you gets a split second less warning — exactly the gap the third brake light was designed to close. Since 1986, every new passenger car sold in the US has included this center-mounted light, and light trucks and vans followed starting in 1994. It sits higher than the corner brake lights, making it visible through the rear window of the car ahead — a small addition that has measurably prevented thousands of rear-end crashes. Here is what it does, why it exists, and what to do if yours stops working.

Why the Third Brake Light Was Mandated

Before the CHMSL, rear brake lights sat in the same housing as turn signals and taillights — roughly waist-high and close together. The problem was that a car in front of you could obscure them completely. The results were dramatic enough that the National Highway Traffic Safety Administration (NHTSA) made it a requirement under Federal Motor Vehicle Safety Standard (FMVSS) No. 108.

How It Works and What the Law Says

The CHMSL is wired to illuminate with a steady, continuous beam whenever the brake pedal is pressed — no pulsing, flashing, or brightness variation is allowed. NHTSA interpretations are clear: pulsing stop lamps violate federal law. The lamp sits centered and elevated, typically integrated into the rear deck, spoiler, or tailgate, and operates on standard 12V automotive voltage (6V units exist for vintage vehicles). It is not a taillight — it only comes on during braking, not when headlights are active.

The regulation also means the light must be visible from behind and unobstructed. Many vehicles do not have a dashboard warning for a failed CHMSL, so routine visual inspection is the only way to catch a burned-out bulb.

What to Do When the Third Brake Light Stops Working

Because there is rarely a dashboard alert, you may not notice a failed CHMSL until someone points it out. Replacement is straightforward on most vehicles. Access the CHMSL assembly from inside the trunk or cargo area — it is usually behind a panel or embedded in the rear deck. Disconnect the wiring harness, remove the bulb or the entire assembly, and install the replacement. Some vehicles use a sealed LED unit that requires replacing the entire light assembly rather than just the bulb.

If you are looking to customize or protect the housing, a 3rd brake light cover can add a subtle style upgrade while keeping the light visible and road-legal.

Do You Legally Need a Working One?

Yes, your vehicle was required to have a CHMSL when it was manufactured — it is mandatory safety equipment under FMVSS No. 108. But enforcement varies by state. While new cars must have one, many US states only require one working rear brake light (the left or right corner light) for a vehicle to pass inspection or be road-legal. A burned-out third brake light can still fail a safety inspection in states that check exterior lighting. The safest approach: test yours monthly by backing up to a reflective surface and pressing the brake pedal, or ask someone to check for you. If it is out, replace it promptly —