During this pot-hole season, I lost the low beam headlights. The light bulbs checked out. On the three pins to the head lamps, I have 12-volts on all three pins? The high-beam works - so I don’t think I lost the ground. Is there a resistor within the headlight switch that could have failed? All the fuses are good as well.
If you are hitting a lot of potholes when you drive…you may be jarring the low beam filaments so hard that they are failing.
Plus, you can bench test the bulbs with jumper wires from the battery.
BUT
Bumps are very bad for bulb longevity . This is why so many bulbs are out on vehicles with the lamps in the lift gate and trunk so often…they get shaken to death.
12 volts on all three pins, you have a problem. If you are measuring this with the bulb plugged in and the lights turned on, you have a bad ground. You will find that the high beams may be on, but they are not as bright as they should be because they are finding a low resistance path to ground through the low beam circuit or the DRL relay.
I expect you measure 12 volts on all pins on the connector you’ve removed, b/c there’s a path through the bulb on the other side that remains connected. To test this, you could remove the other bulb, then I think you’d only measure 12 volts on one pin of the connector.
The headlamp switch probably grounds one or the other of the high and low beam paths, which should illuminate both headlights. Since you lost both low beams, but both high beams work, either the low beam part of the headlamp switch is kaput, you’ve got a bad connection at the low beam terminal of the headlamp switch, or both bulb’s low beam filaments coincidentally burned out at the same time.
If all filaments work, you’d measure a low resistance (a few ohms) between any 2 of the 3 bulb pins. If you measure an open circuit between any 2 of the 3 pins on the bulb, the bulb is burned out.