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Both the headlight switch and the BCM provide a ground, or control circuit, to the headlight relay coil circuit for manual or automatic headlamp operation. When you place the headlamp switch in the OFF position, and the ambient light sensor indicates low light conditions, the BCM energizes the headlamp relay for low beam operation. You may switch the headlights to high beam by pulling the dimmer switch handle toward you until the dimmer switch is activated. When the dimmer switch is placed in the HIGH position, the indicator lamp input to the BCM is pulled low. The BCM then sends a class 2 message to the instrument cluster in order to illuminate the high beam indicator lamp.
Low and High Beams Headlamps Inoperative ?
Replace the headlamp switch. Refer to Headlamp Switch Replacement .
Did you check the headlamp relay ? Location ,left instrument panel
Could be your dimmer switch gone bad, that controls your low and high beams, the contacts for the low beam is usually the one that goes bad first, the high beam contact isn't used much.
My Oldsmobile had the same problem, only high beams worked, replace the dimmer switch and fixed the problem. Unfortunately with my car replacing the dimmer switch gave me a hard time. Most dimmer switches are located in the steering column underneath the dash board and a little hard to get at. Dimmer switch costs about 12 to 15 dollars, very cheap but very hard to reach and replace. The switch it self is activated with a small metal rod going up and down next to the steering column and into your turn signal lever, when you pull up it pushes the metal rod down to activate your high or low beam lights.
Unless both low beam filaments of bulbs have blown at same time I would say you've got a bad switch or a wiring issue. Anything is possible I would check the bulbs first, Look for gray tint in the bulb or broken wire in the bulb. You can always put a new bulb in one side and see if it starts working, if not I would move to the switch.
Move your car to a dark area, such as waiting until night or moving to a dark garage, to test your headlight switch.
2Turn on your car, so you can see your dashboard lights. Note whether some of your dashboard lights fail to illuminate.
Turn on your car's headlights. Note whether your car's headlights illuminate.
4Press the high beam lever to activate your high beams. Note whether your high beams activate.
5Put your car in "Park." Move to the back of the car. Note whether your parking lights are illuminated.
6View any remaining driving lights, such as lights above the license plate. Note whether these lights come on. If more than one set of lights does not activate when you turn on the lights, you may need to replace your headlight switch. If only one set of lights does not turn on, try replacing the lightbulbs in that set and then retesting your car. If the lights still do not illuminate, you may need to replace your headlight switch.