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I have a 2008 Cobalt LS. I was just recently notified by a police officer that my driver side brake light did not work however when the brake pedal is pressed the third and right brake light brightens up perfectly. All light bulbs are on and working when I toggle on the lights for night drive. I also checked to make sure all turn signals work by turning on the hazards and all but the back left one doesn't. All lights assumingly work but the turn signal light in the left rear has no activity whatsoever in my findings...the only bulb. Just to verify I tested the turn signals individually and the right blinks at the standard pace while the left one does it at a rapid succession, also shows on my dashboard. Is it just the signal lightbulb that needs replacing? And will that effect the brake light if there is a bad turn signal bulb?
 

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The taillight/brake light/turn signal is all in the same bulb - it is a 3157, which is a dual filament bulb. Sounds like the high-intensity filament went out, which is why the brake light and turn signal don't work (and why you get a rapid flash on the left turn signal - not enough resistance in the system to maintain the standard flash rate).

You just need to replace the left rear bulb with a new 3157 bulb.
 

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MP81 is pretty accurate here. The BCM has a sense resistor to detect a burnt filament, it's only way to tell the driver that he has a burnt bulb is by doubling the flicker rate whether it be a front or rear blinker filament failure. Same with losing cruise if all 3 High brake light bulbs fail. Same if you lose all 3 rear tail lights either from a burnt fuse or all 3 burnt bulbs, your cluster lighting will go dead. These are smart ways for your car to tell you what's wrong. Or your mechanic if you aren't inclined and didn't read your owners manual from cover to cover.
 
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