Brake lights are a great way to communicate with other road users behind you. They help keep keen drivers from rear-ending you since they light up any time you hit the brakes to slow down your vehicle.
If the lights fail and constantly stay on (or don’t come on when you brake), other road users will have to rely on other cues to figure out when you try to slow down. This is dangerous and could cause accidents, especially at night. Not to mention that driving with faulty brake lights is a traffic offense.
How Do Your Brake Lights Work?
Your brake lights rely on a brake pedal position sensor to determine when your foot is on the brake. The switch is sensitive enough to detect even the lightest application of the brake pedal.
This switch is either an analog position sensor or a digital one used to feed ECU information for other controls requiring the brake pedal to be depressed to work. This includes shifting into gear in an automatic or starting the engine in Push ignition vehicles.
Anything that gets the switch stuck on the engaged position will keep your brake lights on even when you’re not braking.
A Faulty Brake Switch
The brake switch is, in most cases, a plunger-operated switch mounted on the brake pedal under the dashboard. Some wiggle in the mounting harness lets the switch detect when you slightly depress the pedal.
Dirt debris or misalignment could jam the switch open, forcing it to think your brake pedal is constantly engaged.
In some vehicle models, a poorly installed pedal could also mess with the switch, keeping it stuck open. This will keep the brake lights on even when the car is turned off.
While many older cars wire the tail lights straight to the switch, modern cars take it’s input to the ECU through a Controller Area Network (CAN) bus. This makes it easier for the ECU to use the brake position information to control other features in the car.
To guarantee sensitivity, switches in modern vehicles are susceptible and will detect even the slightest pressure. This is enough to bring the brake lights on. Some can also see the gradual position of the brake pedal, but the ECU uses this information for other purposes.
A Stuck Brake Pedal
While a stuck brake pedal is hard to miss in a normal working car, you can easily keep driving if you have a semi stuck pedal and your brake fluid levels are low enough not to transfer the slight depression to your brake pads through the brake booster.
This slightly stuck pedal will nonetheless be enough to keep your brake lights stuck on. Inspect the brake pedal and ensure it is fully retracted when not depressed.
You could also note how your car behaves when you brake. If you experience some slight drop in the brake pedal before it engages, then your braking system is more than spongy, and the mere weight of the brake pedal might be enough to trigger the brake lights by itself occasionally.
Also, if someone recently tinkered with your brake pedal, they might have inadvertently installed the switch wrong or messed up with the pedal, leaving it partially stuck.
ProTip: Riding the brake pedal could also simulate a jam keeping your brake lights constantly on. This is distracting to drivers behind you and negates the purpose of brake lights. It will also ruin your brake pads and rotors. Keep your foot off the brake pedal if you are not using it.
Problems With Your Brake Lights Wiring
The last possible reason your brake lights don’t turn off is a problem with the wiring. The issue could be between the brake switch and the control module or between the control module and the brake lights.
Frayed switch wires could come into contact sending an ON position signal to the control module. Since the CAN bus and control module signal wires don’t transmit much current, this could go unnoticed as it won’t cause any sparks or burning smells.
A fault between the positive line to the brake pedal is harder to simulate. Frayed wires often short to ground. It is hard, though not impossible, to have the positive lead of your brake lights grab current from another nearby damaged wire.
Faults in the wiring system are more common in older vehicles or if you modified the OEM wiring harness to fix a problem or modify your vehicle’s electrical system.
An Issue With the CAN Bus
If you drive a modern (after 2012) model, chances are the signal goes through the CAN bus and control modules. The CAN bus system and controller modules within it can fail. It can be a software or a hardware problem. You will need an expert mechanic to diagnose and fix the problem.
How to Check If Your Brake Lights are Working Right
The easiest way to tell if your brake lights are working fine is to have someone stand behind the car and check your brake lights as you operate them. Have them confirm that all three (or more) brake lights
If you are alone, park in front of store windows or your garage wall with the lights off. You will see the lights glow and illuminate the wall when you step on the brake pedal and shut off when you lift off.