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If you have the standard halogen lights, then the main headlight has 3 bulbs aside from the fog light and turn signal; 1) low beam, 2) high beam, and 3) a small running/parking light that comes on with the headlight switch in the parking light or headlight on position. I'm not sure about the setup in the hid litronic lights, but I know that they also have a small running/parking light bulb that is on with the headlight switch in any on position.

With this running light on, it looks like the headlight is on but dim.

Since it's the easiest to do, I would suggest checking the bulb and the headlight-to-car connector first.
Assuming that the high beam works, try putting the high beam bulb in the low beam socket if they are OEM halogen(they should both be identical 55 watt bulbs) and see if it solves the issue. If so, replace the bulb. If not, check the connector next.

FYI: when looking at the headlight from the rear with the rear cover removed, the low beam is on top, the high beam is on bottom, and the running/parking light is off to the side.