Cadillac CTS Brake Light Fail - Common Causes And Solutions

By Nick Marchenko, PhD

If you own a Cadillac CTS and encounter an issue where your brake lights are not illuminating properly, this guide is designed to help you diagnose and resolve the problem effectively.

Step 1: Preliminary Checks

Check the Fuses: Start by inspecting the fuses related to the brake lights. Refer to your owner's manual to locate the correct fuses and check if they are intact.

Inspect the Bulbs: Although your tail lights may light up, the brake light section of the bulb could still be faulty. Remove the bulbs and check for any signs of damage or an open filament.

Planning to replace a light bulb, but don't know which one you need for the brake light of your Cadillac CTS? Find out here:

• 2014;
• 2013;
• 2012;
• 2011;
• 2010;
• 2009;
• 2008;
• 2007;
• 2006;
• 2005;
• 2004;

Step 2: Relay Inspection

• Locate the Relay: Some Cadillac CTS models have the brake light relay located under the back seat on the driver’s side. You may need to refer to your vehicle’s manual or an online forum for the exact location.
• BAS Relay and Fuse: Often, the problem lies with the BAS (Brake Application Sensor) relay or fuse. These can be purchased at any auto parts store and are relatively easy to replace.

Step 3: Advanced Troubleshooting

• Third Brake Light Issues: In some instances, the third brake light transformer might fail, affecting all the brake lights. Consider replacing this with an LED unit for better reliability.
• Switch on the Pedal: There’s a possibility that the switch on the brake pedal is at fault. This requires checking the continuity of the switch when the pedal is pressed.
• Testing Voltage and Ground: Access the rear lamps and check the voltage at the white wire with the brake applied. Also, verify the grounding on the black wire. The high lamp and rear lamps do not share the same ground but do share the same voltage.

Step 4: Wiring Inspection

• Harness and Connectors: Inspect the wiring harness, especially where it bends near the trunk, as wires can break due to frequent bending. Look for signs of wear or breakage.
• Test Light Use: Utilize a test light to check for power at the brake light connector. If there’s no light, it could indicate a broken wire or a bad connection.

Step 5: Professional Diagnosis

If all the above steps do not resolve the issue, it is advisable to seek professional help. A certified mechanic or a dealer can use diagnostic equipment to trace the problem, which might be related to the car’s computer system.

Preventative Tips

• Regularly check your brake lights to ensure they are working correctly.
• When replacing any electrical components, ensure they are compatible with your Cadillac CTS model.
• Consider using higher quality or LED bulbs for longer life and better performance.

In conclusion, fixing a brake light issue in your Cadillac CTS can range from simple DIY tasks like checking fuses and bulbs to more complex issues like wiring harness problems. If you are not comfortable performing these tasks, seeking professional help is always the best course of action. Remember, functioning brake lights are crucial for your safety and the safety of others on the road.

See also:

• Why is my ABS Light on in my Cadillac CTS - Causes and How to Fix

Nick Marchenko, PhD

Industrial Engineer & Automotive Content Specialist

Nick writes in-depth guides on car clubs, engine specs, vehicle ownership, and modifications, combining engineering knowledge with automotive passion.

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