Back To Blog Academy

Understanding the Brake Light Switch - A Crucial Element in Vehicle Safety

By R&D
Published on January 5th, 2024

As an Amazon associate, we earn from qualifying purchases.

Few components are as essential to driving safety as the Brake Light Switch. This part plays a pivotal role in ensuring the safety of the vehicle and its occupants. This article outlines operation of the Brake Light Switch and its importance to vehicle operation, identifies symptoms of a faulty switch and provides troubleshooting and maintenance recommendations.


The Brake Light Switch, commonly known as the BLS, is a fundamental component in modern vehicles. Its primary function is to activate the brake lights when the brake pedal is pressed. This action alerts drivers behind the vehicle that it is decelerating or stopping. This indication is critical to preventing rear-end collisions. In addition to this crucial safety function, the Brake Light Switch also plays a role in disengaging cruise control systems and activating the shift interlock.

Brake Light Switches come in various types, but the two primary types are:

  • Mechanical Brake Light Switch: This traditional type of BLS is mounted near the brake pedal, and is activated through a mechanical connection when the brake pedal is pressed to trigger the brake lights.
  • Hydraulic Brake Light Switch: This switch type is commonly found in newer vehicles. The hydraulic BLS is integrated into the brake hydraulic system. It uses changes in hydraulic pressure to activate the brake lights, providing a more accurate and immediate response to brake application.

Symptoms of a Faulty BLS

A malfunctioning BLS can lead to various safety and operational issues in a vehicle. Common signs of a faulty BLS include:

  • Inoperative Brake Lights: The most obvious sign of a faulty BLS is that the brake lights do not illuminate when the brakes are applied, which can lead to rear-end collisions.
Understanding the Brake Light Switch image 1.jpg__PID:f5affd66-bf40-4e5b-89d7-85e5aef34959

Brake-Shift Interlock Issues

  • Stuck Brake Lights: In some cases, the brake lights may remain on, even when the brakes are not engaged. This condition can potentially drain the battery and may cause confusion for other drivers who will assume the vehicle is slowing or stopping.
  • Cruise Control Inactivity: A malfunctioning switch may prevent the deactivation of cruise control when the brakes are engaged.
  • Brake-Shift Interlock Issues: Some modern vehicles use the BLS to activate the brake-shift interlock, preventing the driver from shifting out of "Park" without engaging the brakes. A faulty BLS can lead to problems with this system.

Possible Causes

Several factors can lead to a malfunctioning Brake Light Switch:

  • Mechanical Wear: Over time, mechanical components in a BLS can degrade, leading to diminished performance or failure.
  • Hydraulic Leaks: In hydraulic BLS systems, a leak or loss of pressure in the brake hydraulic system can cause the switch to malfunction.
  • Electrical Issues: Faulty wiring, blown fuses, or relay problems can affect BLS operation.
  • Contamination or Corrosion: Exposure to moisture, dirt, or corrosive substances can impair switch functionality.

Find the perfect scanner in 1 minute

Diagnostic Tips

When a faulty Brake Light Switch is suspected, follow these steps to confirm the issue:

  • Visually inspect the switch and its wiring for visible physical damage, such as frayed wires, loose connections, or corrosion.
Understanding the Brake Light Switch image 2.jpg__PID:4de84c24-5672-44ee-ab1f-2dae0ba7d32c

Check for Physical Damage

  • Use a multimeter to check for continuity in the switch's circuit within the specified voltage range when the brake pedal is pressed.
Understanding the Brake Light Switch image 3.jpg__PID:7f022bab-edf1-4e0b-a134-174549a4ab41

Check the Switch Circuit

  • Use a Diagnostic Tool (such as an OBD2 Scan Tool) to check for Diagnostic Trouble Codes (DTCs) related to the Brake Light Switch.
  • Consult the vehicle’s Service Manual for specific diagnostic procedures and recommended troubleshooting steps.
Understanding the Brake Light Switch image 4.png__PID:68fe3ea5-8551-4e62-8407-5b1171a63d5a

Innova Tool Checking DTCs

Cost of BLS Replacement

Replacing a malfunctioning BLS is typically a straightforward procedure, with costs ranging from $10 to $50. However, prices may vary depending on factors such as the vehicle’s make and model, as well as regional labor rates.

Understanding the Brake Light Switch image 5.jpg__PID:720cd6dd-4a8d-40b9-9cbd-f51f3f61ca17

BLS Replacement


The Brake Light Switch is a critical component in ensuring road safety. Its role in alerting fellow drivers that a vehicle is slowing or stopping is paramount in preventing rear-end collisions. Its presence underscores the importance of even the simplest parts in the ever-evolving automotive landscape.

Back To Blog Academy

Have Questions?

Hop in the discussion board on our community site!
It's a place where we help each other answer questions. Like Reddit but for automotive lovers.

Visit Community