Hid lamps operate with a constant power or constant current function.

As the lamp wears it will take too much voltage to supply the current desired or the current will get too high to supply the power desired, the ballast will shut off if either voltage or current gets out of bounds.

Bulbs get a LOT dimmer before they fail and it's so slow you don't notice: a prime reason why to replace both at once. Usually you'll notice some color shift, that's when to replace them.

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2011 E70 • N55 (me)
2012 E70 • N63 (wife)