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I have a 2012 Ford Focus (sedan version). The third brake light stopped working recently and I'm trying to determine if the problem is with the light itself or inside the vehicle, but a I have very limited toolset.

Is there anything I'd have laying around the house that I can use to either confirm that the light works by itself (it says 13.5v and has two pins that the wires connect to) or something else I could (safely) wire to the car instead of the light, that would identify where the problem is?

Light Location

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  • straighten the pins on a 194 bulb and use it as a test light. or just start smacking the light till it starts working again/breaks. – Ben Mar 16 '17 at 0:45
  • By "third brake light," do you mean the Center High Mount Stopping Light (CHMSL)? It's located near the rear window along the vehicle centerline? – Hari Ganti Mar 16 '17 at 0:53
  • @HariGanti - Yes, I believe that is it. – Rob P. Mar 16 '17 at 2:21
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I'm assuming you have the LED array in the middle. If you can get to the wiring and disconnect it, you could use a simple 9 volt battery to test the array. Make sure that you don't reverse the wiring - I'm not sure what kind of reverse bias protection the array has. It should light up - a little dim, but enough. If you have to extend the existing wiring, use temporary speaker wire or similar and try to plug it in on the BACK of the plug, don't push anything into the front as you may damage the small connectors.

To test the power coming from the brake switch, wedge it down and use any of the following - a multimeter, a 12 volt probe (cheap), a neon or led bulb (some are designed for 12 v), a bulb from the auto store - look for one with pins, a cheap led from wherever (you'll instantly blow the LED, but you'll at least know you have power)...

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  • This is great. Thank you. I do have a 9v battery and I've already managed to get the light out of the car - I'll see if I can find a wiring diagram or something that tells me which lead should be + and -. – Rob P. Mar 16 '17 at 1:37
  • Hooking it up backward shouldn't harm anything. LEDs have inherent reverse bias protection. – Hari Ganti Mar 16 '17 at 2:29
  • Based on the thing not lighting up with the battery, I concluded the light itself was broken. Order the replacement. Plugged it in today and it works great. – Rob P. Mar 18 '17 at 16:01

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