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HID bulbs don't dim over time like halogen bulbs simply because they don't have a filament. HID bulbs produce an arc. As a filament deteriorates, it deposits a metallic film inside the bulb. That's what reduces light output. That doesn't happen with HID. HID systems have an ignitor, ballast and bulb. The ignitor generates very high voltage to start the arc....


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From a circuit diagram I have found on the internet (if it is correct for your car), it looks like there is a pair of headlight relays in a fuse box under the hood. These relays switch the +12V to the headlights. It looks to me that if one of the relays fails, then the +12 will find its way through all 4 bulbs making them all light when switched to high ...


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