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I've got a 2008 Chevrolet Cobalt. The passenger front blinker, passenger rear blinker & brake light, and the middle brake light are all out. The hazard lights also do not work on the passenger side.

I've looked at all the fuses I can find that remotely deal with lights in the floor panel & under the hood; no visual issues. I've looked at all the bulbs themselves, and none of them seem busted.

Does this sound like an electrical short somewhere? Maybe the turn signal switch? I don't really know where to start or where to look to diagnose the issue. Any thoughts?

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  • Welcome to Motor Vehicle Maintenance & Repair! I'd think the switch is suspect as well, but off the top of my head I couldn't tell you how to test it.
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Sep 28, 2018 at 0:38
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    The rear blinker and brake light probably have the same "earth" return circuit, if they are in the same lamp cluster, even though the switches are different for the brake and indicator lights. That could be a clue as to where to start looking - find where the return circuits are not bonded to the car, or the battery. (I've never seen any sort of Chevy in the UK, so no model-specific advice!)
    – alephzero
    Sep 28, 2018 at 1:04
  • Do the park lamps work? How about the reverse? Is this a coupe or sedan? Have you checked the bulbs?
    – Ben
    Sep 28, 2018 at 1:30
  • @Ben Yes, park & reverse lamps work. 4 door. I've checked the bulbs, and swapped the left (working) to the right, and it doesn't work, so I think the bulb is ruled out.
    – Damon Bauer
    Sep 28, 2018 at 14:59

1 Answer 1

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Take a look at the wiring diagram below. The important clue is you said the middle brake light isn't working.

The CHMSL (Center High Mounted Stop Light) works only from closing the dedicated relay. This signal provides power to the lamp, and also instructs the engine and transmission control modules that you are braking.

Unfortunately, this function is provided solely from the BCM (Body Control Module). If that module is bad, it can affect many things - including the CHMSL and the passenger side signals.

If the module is indeed bad, it will be an expensive repair - and involve a special "Tech" GM tool to program your vehicle's specific information.

Alephzero's comment is a good one; I would check the shared ground paths (you can see them as dotted lines in the diagram). However, I doubt the FRONT and the rear share the same ground path, which leads me back to a BCM issue.

Identify the CHMSL relay in the underhood fuse block, and have someone feel it while you pump the brakes with the car on (not running). It should click everytime you hit the brakes. If not, that's bad news and points to a bad BCM.

Good luck.

enter image description here

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  • Thanks @SteveRacer. I'll take a look at the CHMSL relay as you suggested and see what I can find.
    – Damon Bauer
    Sep 28, 2018 at 13:00
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    @motoxer4533 the oe stop lamp circuit diagram doesn’t show the whole picture. Tail/turn/stop/marker/chmsl and reverse in the right side all share a common ground. G402 in that diagram, it’s located near the speaker on the shelf.
    – Ben
    Sep 28, 2018 at 13:49
  • @Ben - that might be great news; far less a problem than a virgin BCM. I wonder why my AllData diagram shows G402? In any case, the control side of the CHMSL relay is G301, so it's still a good troubleshooting step.
    – SteveRacer
    Sep 29, 2018 at 1:21

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