The third brake light bulb is called the center high mounted stop lamp (CHMSL) bulb, commonly referred to as the CHMSL or the third brake light. It sits at the center of the rear of the vehicle and signals braking from a higher position than the standard taillights.
Terminology and purpose
In automotive terminology, the third brake light is officially known as the Center High Mounted Stop Lamp (CHMSL). The name reflects its elevated placement and its function as a supplementary brake signal, designed to improve visibility to drivers behind you.
Origin and regulatory role
The CHMSL concept was introduced to enhance rearward visibility and reduce collisions. In the United States, regulations based on FMVSS 108 require a center high mounted stop lamp on new passenger vehicles, a standard that has been in place since the mid-1980s. Other regions have adopted similar requirements to improve traffic safety.
Bulb types and replacement considerations
Today, CHMSLs come in two common formats: traditional replaceable bulbs behind a lens or integrated LED modules. Older vehicles typically used incandescent bulbs, while most modern cars use LED assemblies that may be sold as a complete module rather than a single bulb.
Incandescent vs LED CHMSL
Incandescent CHMSL bulbs are individual filaments in a socket that can be replaced if the housing permits. LED CHMSL modules are usually a single, sealed unit or a compact LED strip assembly; when an LED module fails, owners often replace the entire CHMSL housing rather than a single component. Always consult the owner's manual for the exact replacement method for your vehicle.
Maintenance and replacement steps
If you need to replace the CHMSL, here are typical steps. Note that the exact procedure varies by vehicle model, so check your manual or consult a professional if in doubt.
- Identify whether your CHMSL uses a replaceable bulb or a sealed LED module by consulting the owner's manual or inspecting the unit.
- For replaceable bulbs: disconnect the battery, remove any access cover from the trunk or rear hatch, twist or unclip the bulb socket, replace the bulb with a correct type, reseat the socket, and test the light before reassembling.
- For LED modules: often you must replace the entire CHMSL assembly or module; unplug the connector, remove mounting screws, replace with a compatible LED unit, reconnect the wiring, and test the light.
- After replacement, test the CHMSL by pressing the brake pedal with the headlights on (or in a dark area) to confirm visibility from a distance.
If you’re unsure about the process or cannot locate the correct replacement part, seek help from a dealership or a qualified auto technician. CHMSL maintenance is generally straightforward but varies by vehicle design.
Regulatory context
Beyond its name and function, the CHMSL is a safety feature anchored in regulatory standards. In the United States, federal safety requirements under FMVSS 108 mandate the presence of a center high mounted stop lamp on applicable new vehicles, with regional and national variations in implementation across markets.
Practical considerations for owners
Owners should consider compatibility, part availability, and ease of replacement when dealing with CHMSL issues. LED modules tend to last longer but can be more expensive to replace, while incandescent bulbs are cheaper but may require more frequent maintenance. Regular checks during vehicle servicing help ensure the brake signal remains reliable.
Summary
The third brake light is officially the Center High Mounted Stop Lamp (CHMSL). While older vehicles used replaceable bulbs, modern cars more often use integrated LED modules. The CHMSL serves a critical safety role by providing a high-visibility brake signal, a standard upheld by regional regulations since the mid-1980s. For maintenance, consult your manual to determine whether you need a bulb or a whole LED module and follow proper safety steps when replacing.