GMC Sierra Third Brake Light Not Working – Causes & Fixes

The GMC Sierra boasts three brake lights, including two on either side and the other in the center. Typically, the third brake light offers an extra signal to absent-minded drivers. However, like the rest of the lights, the GMC Sierra’s third brake light may suddenly stop working.

But what causes your GMC Sierra third brake light not working? It can be due to a blown fuse or a short circuit. Sometimes, it can be a burned-out bulb caused by moisture in the housing assembly. Other causes include a defective brake light switch and poor ground wire.

So, how do you fix problems with the Sierra’s third brake light? In this guide, you can find all the answers to your questions. 

Causes and Solutions for GMC Sierra Third Brake Light Not Working Table

In a hurry? Here is a quick overview of the causes of Sierra third brake light illuminating and the possible solutions. 

Causes	Possible Solutions
Short Circuit	Splice back together the shorted wire or replace wire entirely.
Blown Fuse	Install a new fuse.
Burned Out Bulb	Replace the bulb or the entire third brake light assembly housing.
Faulty Brake Light Switch	Install a new switch.
Bad Electrical Ground	Tighten any loose connection and clean the corroded ground wire.

Causes of GMC Sierra Third Brake Light Not Working with Solutions

Sometimes, your GMC Sierra center high mount stop light or third brake light may stop working out of nowhere. The causes of this include the following:

1. Short Circuit

A short circuit in the electrical wires is one of the reasons the third brake light on your GMC Sierra may stop working. Sometimes, it could just be one burnt wire. You can tell if there is a short in the circuit if you replace the fuse and it blows again.

So, in other words, a short circuit generally causes the fuse to blow. And this leads to the center high-mount stop light not working on your Sierra. Alternatively, you can use a digital multimeter to test a short circuit. And if you get a high current reading, you have shorted wire.

Solution

1. To fix a short circuit, open the third brake light housing assembly. 
2. Then, use electrical tape to locate the shorted wire and splice it back together. 
3. If this does not work, replace the entire wire and reinstall the fuse. 
4. Next, test the circuit for resistance with a multimeter. If you get a reading of 1 or OL, there is a short circuit in the electrical circuit.

2. Blown Fuse

If all the brake lights are not working, the probable cause of the third brake light not working could be a blown fuse. The fuse can blow due to corrosion in the wiring or a short circuit caused by water.

Generally, the 10-amp fuse under the hood is the one notorious for blowing and causing the third brake lamp to stop working.

Set your multimeter on the ohmmeter setting to check if the fuse is blown. Then, place the multimeter’s positive and negative probes across each fuse’s end. If the reading is “OL”, the fuse is bad. Alternatively, you can visually inspect the fuse for signs of physical damage like broken or melted wire.

Solution

With a blown fuse, the only solution is to replace it. Here is how to complete the replacement:

Step 1: Prop open your GMC Sierra’s hood.

Step 2: Find the engine fuse box on the driver side of your Sierra and remove the cover.

Step 3: Locate and remove the blown fuse from the fuse box.

Step 4: Install the new fuse and secure the fuse box cover. The new fuse should be of the same amperage as the old one. 

3. Burned Out Bulb

A bulb powers the third brake light. These brake light bulbs typically last up to 4 years or 40,000 miles. However, sometimes the bulb may blow up when you least expect it. This happens when your Sierra third brake light assembly has cracks, allowing water to sip inside.

Solution

The best solution is to replace the burnt-out bulb. However, if the brake light assembly has moisture, you must replace it to prevent the bulb from blowing up again. Here is how to go about it:

Step 1: Get a Phillips screwdriver and unscrew the 2 screws securing the third brake light housing assembly.

Step 2: Wiggle the connector to disconnect the wiring harness from the brake light.

Step 3: Disconnect the connections to the light bulb from the housing assembly. Then, twist the bulb counterclockwise to remove it from the socket. 

Step 4: Next, install the new bulb into the socket and the housing assembly. Then, reconnect the connections to the bulb and wiring harness. 

Step 5: If the light assembly is cracked, skip steps 3 and 4 and install the new assembly. 

If you are replacing the entire housing assembly of your third brake light, you can watch this video for a detailed illustration:

4. Faulty Brake Light Switch

The right and left brake lights of the GMC Sierra are wired through the same brake switch as the third brake light. Therefore, if all the 3 brake lights are not working, the switch is faulty. This is because the switch tells the brake lights to turn on when you press the pedal.

Generally, an electric issue or problem with the brake light circuit can cause a faulty brake light switch.

Solution

First, use a multimeter to test the brake light switch to confirm it is faulty. Simply set the multimeter to “Ohm meter”. Then, hook the two probe leads to the brake light switch terminals and check the reading. If you get a 1 or OL reading, the switch is faulty, and you must replace it.

Here is how the replacement is done:

Step 1: Locate the brake light switch near the top of the pedal arm just under the dashboard.

Step 2: Unplug the wiring harness from the brake switch.

Step 3: Use a 10mm ratchet or wrench to unfasten the bracket securing the switch.

Step 4: Get a flathead screwdriver and pop off the clip of the brake switch.

Step 5: Pull out the brake light switch and install the new one in reverse order.

5. Bad Electrical Ground

If the brake light switch is working, but the center high mount stop bulb is not, the ground wire could be bad. To confirm this, inspect the electrical ground to which the 3rd brake light is connected for corrosion or loose connection.

Solution

For a loose connection, you simply retighten the ground wire. However, if the ground wire is corroded, you must clean it, and here is how to do it.

Step 1: Remove the third brake light housing assembly.

Step 2: Locate the ground wire for the center brake light and disconnect it from the connector. It should be black in color.

Step 3: Clean the connectors, frame, and ground wire using sandpaper or wire brush.

Step 4: Attach the ground wire to the connector and ensure it is tightly fastened.

Step 5: Reinstall the third brake lamp assembly and test the light. 

When dealing with brake light malfunctions in your GMC Sierra, it’s worthwhile to be aware of similar problems in other vehicles. The Jeep Gladiator’s 3rd brake light not working is a scenario that has left many drivers searching for answers. Additionally, the Chevrolet Silverado, another popular truck, has had its share of issues with the Silverado 3rd brake light malfunctioning. Exploring these topics will arm you with a broader understanding of brake light concerns across different vehicle models.

FAQs

In this section, we will briefly respond to frequently asked questions about the center high mount stop light not working.

Final Words

The third brake light of your GMC Sierra is just as important as the rest of the brake lights. Therefore, if it is not working due to a shorted wire, burned bulb, or blown fuse, you should have it fixed immediately. If not, you could get in trouble with the law enforcement officers.

But don’t fret because fixing a faulty center brake light is an easy task if you have basic mechanic skills. In most cases, you just need to replace the defective component, such as the bulb, fuse, or brake light switch.