Brake Lights Aren’t Working on this 2021 Chevy Express: Diagnosis and Quick Fix

Today’s customer rolled into the shop complaining that the right-rear light on his 2021 Chevy Express was not working: the turn signal, marker light, and brake light were not lighting up.

I started this fix by pulling up a wiring diagram for this 2021 Chevy Express. In cases like this, looking for diagnostic trouble codes (DTCs) is an unnecessary step, because the most likely cause is a blown-out bulb or a faulty or failing electrical component.

According to the wiring diagram for this van, there are two separate circuits controlling the taillight assembly. One circuit controls both the turn signal and brake light, while the other circuit controls the running light.

Next, I checked the bulb. It was damaged and slightly burnt on the inside. After replacing it, the light still did not work, so it was time to disconnect the wiring from the assembly to test for good current. I unplugged the connector for the tailight assembly and used my power probe to check for power and ground. I found no power, but the assembly had a good ground.

I decided to check both fuses connected to this taillight assembly. and found that both were blown. These fuses can be found in the fuse panel under the driver’s seat. Once the fuses were replaced, the running light came on, but the turn signal was still not working.

According to my wiring diagram, the turn signal and brake light were controlled by the body control module (BCM). With good fuses and a new bulb, the BCM was the next place to check. Using my multimeter, I probed the left-turn…