Third brake light always on

[jhs914]

I pulled in driveway yesterday in my wife's car and noticed that the third brake light on the back of the cab on my 2017 GMC Canyon was on. The 2 regular brake lights were not on and they work normally. I haven't had time to try and diagnose the problem. I have a multimeter and a test light but no other diagnostic equipment. Has anyone else ever had this problem or have any ideas where to start?

 

[paulo57509]

I wonder if the high mount is wired thru the stop light switch? I'd start here.

 

[dishdude]

This circuit is certainly controlled through the BCM, there might be a code set.

 

[jhs914]

I wonder if the high mount is wired thru the stop light switch? I'd start here.

The regular brake lights are working properly so unless there is a separate brake light switch for the third brake light that is not the problem.

 

[Junk]

All of the rear light wiring on this truck goes directly back to the BCM, meaning the brake pedal sensor or switch sends a signal to the BCM to turn the lights on. Also, the center brake light is on a completely separate circuit, controlled by a relay in the underhood fuse panel. You could have a short to power or an internal BCM problem, which are both highly unlikely, but my bet is that that relay is stuck. I'd try getting into the underhood fuse panel and swapping the center high mount stop lamp relay with another identical relay in the box and see what happens. If the light works normally with a different relay then it's stuck on.

 

[208vette]

Had this issue when I went to the Oregon Coast for a week in the corvette. Third brake light started to stay on dimly after being there for a couple days. Apparently an issue in high humidity areas. Condensation gets inside the the main connector that branches off to all the taillights causing the light to be on dimly. Dried out the connector and put some dielectric grease in it and no more issues.

 

[jhs914]

All of the rear light wiring on this truck goes directly back to the BCM, meaning the brake pedal sensor or switch sends a signal to the BCM to turn the lights on. Also, the center brake light is on a completely separate circuit, controlled by a relay in the underhood fuse panel. You could have a short to power or an internal BCM problem, which are both highly unlikely, but my bet is that that relay is stuck. I'd try getting into the underhood fuse panel and swapping the center high mount stop lamp relay with another identical relay in the box and see what happens. If the light works normally with a different relay then it's stuck on.

Thanks, I'll try that tomorrow. I took the brake light lense off and unplugged the light to keep from running down the battery.

 

[jhs914]

All of the rear light wiring on this truck goes directly back to the BCM, meaning the brake pedal sensor or switch sends a signal to the BCM to turn the lights on. Also, the center brake light is on a completely separate circuit, controlled by a relay in the underhood fuse panel. You could have a short to power or an internal BCM problem, which are both highly unlikely, but my bet is that that relay is stuck. I'd try getting into the underhood fuse panel and swapping the center high mount stop lamp relay with another identical relay in the box and see what happens. If the light works normally with a different relay then it's stuck on.

I gave the relay a good whack and the light now works properly. I'll pick up a new one and replace it since it may be on the way out.

 

[Hermann]

Have been behind several newer GM pickups with burned out tail lights. Geez 40 years and still tail light issues. I have found it to be the easiest way to quiet a GM fanboy is, when they start crapping on Fords is to mention GM tail light issues. Oh ain't this fun.

 

[Junk]

I gave the relay a good whack and the light now works properly. I'll pick up a new one and replace it since it may be on the way out.

Repair method "A", I like it! Glad you got it figured out!

 

[jhs914]

New relay. Problem solved. Thanks Junk.

 

[mrbreezeet1]

All of the rear light wiring on this truck goes directly back to the BCM, meaning the brake pedal sensor or switch sends a signal to the BCM to turn the lights on. Also, the center brake light is on a completely separate circuit, controlled by a relay in the underhood fuse panel. You could have a short to power or an internal BCM problem, which are both highly unlikely, but my bet is that that relay is stuck. I'd try getting into the underhood fuse panel and swapping the center high mount stop lamp relay with another identical relay in the box and see what happens. If the light works normally with a different relay then it's stuck on.

Good Call!

 

[OilUzer]

Is there a DOT safety requirement to have a dissimilar logic/circuit to control the 3rd brake light?

 

[mk378]

No there isn't but on a GM truck where the main brake lights are the same bulb filaments as the turn signals, there needs to be some logic involved.