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My wife has a 2007 Kia Rio. She started it the other day and when she put her foot on the brake a dashboard indicator about a burnt out light came on.

We looked at it together and found that with the headlights off, only the right brake light came on. Not the left, not the center. With the headlights on and the brake off, both tail lights worked (same illumination level as stepping on the brake with headlights off). But then you step on the brake and the right one gets further illuminated while the left stays the same.

So it doesn't seem like the left bulb is out, because it illuminates normally in response to headlights. But I feel like if it were a switch or something then the right light would behave like the others.

Maybe it's a coincidence that the center light also went out, and I just need to replace the bulb, along with the left light assembly? I don't know. Any ideas?

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  • The bulbs are stop/tail and the left stop filament has blown - replace it.
    – Solar Mike
    Aug 20, 2019 at 4:54

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I think that your left and center brake filaments have blown. The left and right bulbs will have 2 filaments, one for the tail (side) light and one for the brake light. Only the brake filament will have blow in the left one, so allowing the tail light to continue to work. When this brake filament broke, the body control unit (BCU) recognized it and lit the warning light. The center brake light may have been broken for a while, but it probably isn't monitored by the BCU.enter image description here

Stop/Tail bulb image from RS online

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  • So what should the OP do now?
    – Solar Mike
    Aug 20, 2019 at 8:10
  • @SolarMike I don't know how to change a rear bulb on a 2007 Kia Rio, maybe you can suggest. I also think that the OP will be intelligent enough to come to the conclusion that the bulb needs replacing if it has blown.
    – HandyHowie
    Aug 20, 2019 at 8:16
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    Just thought you could have added a simple "replace it" much the same as in my comment above...
    – Solar Mike
    Aug 20, 2019 at 8:17
  • Replacing the bulb is definitely something I can do. It was the fact that the center was also an issue that had me asking. We probably won't get a chance to fix it until this weekend. Will let you know how it turns out.
    – Matt
    Aug 20, 2019 at 12:16
  • It was indeed a simple blown filament that caused the light to come on. I bought the bulb and in 10 seconds had the problem fixed. The center is still out but that doesn't cause the dash light to come on. It was probably out for a long time. Will replace that soon. Thanks.
    – Matt
    Aug 30, 2019 at 3:21
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These symptoms are caused by a bad ground connection, probably on the car's left side at the rear.

I am unfamiliar with this vehicle, but I would find, then open and clean, the ground connections at both rear taillight assemblies.

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  • Pardon my ignorance, knowing very little about cars... But I am an electrical engineer. Could you possibly explain a bit more, at a technical level how a bad ground connection can do this? From what I can tell there is a single bulb for both tail light and braking, it just gets brighter when both are on. I expect the applied voltage simply increases when both are activated. But only the headlight switch causes the left bulb to light up, not the brake. So the ground (for the bulb at least) is fine. I could see this affecting the center light though. Also why the ground and not power? Thanks.
    – Matt
    Aug 20, 2019 at 3:06
  • I'll try! Tail light bulbs usually have two filaments: one for the tail light, and the other for the brake light. A bad ground causes voltage that's supposed to pass through one filament and then go to ground, to instead not be able to pass through the ground connection, and instead find a higher-resistance path to ground back through the other filament in the bulb, or another bulb connected to the same circuit. When a bulb is supposed to go on,the usual symptom is it doesn't but another bulb or filament glows faintly. A search for "bad taillight ground" will generate lots of discussions.
    – DavidRecallsMonica
    Aug 20, 2019 at 3:28
  • Each bulb-to-socket connection, socket-to-wiring connection, and wiring-to-ground-post connection must be checked; high resistance in any could cause what you're experiencing.
    – DavidRecallsMonica
    Aug 20, 2019 at 3:29
  • Thank you. I'll take a look again tomorrow. In my memory there was only a single filament that got brighter but it may have been an illusion. I'm sure I could look up the bulb and find out for sure.
    – Matt
    Aug 20, 2019 at 3:29
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    @Matt The car electrical system is low voltage and the return paths for all the low-current circuits are through the metal structure of the car body, not through separate wires. If a fault develops somewhere there is often an alternative return path which works "sometimes" depending on the position of other switches in the circuit. If the alternative path for a relatively low-current tail light is actually going through the filament of another higher current (and lower resistance) bulb, you might not notice that the tail light is a bit dim and the other bulb is very dimly lit.
    – alephzero
    Aug 20, 2019 at 11:48

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