Why would the turn signals work but not brake lights on my 2003 Chevy Traliblazer?

There are several reasons why your 2003 Chevy Trailblazer's turn signals might work but not the brake lights. The problem likely lies in the brake light circuit, not the turn signal circuit, since they're separate systems that only share some components (like the bulbs and wiring harnesses in some areas). Here are some possibilities:

* Burnt-out brake light bulbs: This is the simplest and most common cause. Check all brake light bulbs (both rear and high-mounted center brake light, if equipped). Even if one bulb is working, the others could be burned out.

* Blown fuse: Your Trailblazer has separate fuses for the brake lights and turn signals. Consult your owner's manual to locate and check the correct fuses.

* Brake light switch: This switch is activated when you press the brake pedal and completes the circuit to illuminate the brake lights. A faulty switch is a very common cause of this problem. It's relatively inexpensive and easy to replace.

* Wiring issues: A break or short in the wiring harness leading to the brake lights could be the culprit. This is harder to diagnose and may require tracing the wires. Corrosion in connectors is a possibility, especially in older vehicles.

* Brake Pedal Position Sensor (if equipped): Some vehicles use a sensor to detect brake pedal position. A fault here might prevent the brake lights from activating.

* Centralized Body Control Module (BCM) or related control module: While less likely, a problem with the BCM or a related control module could cause this issue. This is a more complex repair usually requiring a professional diagnosis.

Troubleshooting Steps:

1. Check the bulbs: This is the first and easiest thing to do. Replace any bulbs that are burned out.

2. Check the fuses: Use a test light or multimeter to verify the brake light fuses are intact.

3. Inspect the brake light switch: This is usually located near the brake pedal. You may be able to visually inspect it for damage or test it with a multimeter (consult a wiring diagram). Replacing it is a relatively easy DIY repair.

4. Visually inspect the wiring: Look for any obvious damage, corrosion, or loose connections in the wiring harness leading to the brake lights.

If you've checked these things and the problem persists, it's best to consult a mechanic or auto electrician. They have the tools and expertise to diagnose more complex electrical issues, such as problems with the BCM or wiring harness.