With an inexpensive volt-ohm-meter, you can debug most (halogen) headlight glitches. First, remove the lo-beam bulb and test it for continuity - a good bulb will elicit a beep. Next, test the harness wire pair (bulb out) for volts -- should be around 13V dc when the lo-beam on the other side is lit. If close to 0 volts, you next want to test the ground wire for continuity. Lights off, measuring ohms, one of the two wires should have a solid connection to ground (any bare metal bolt head on engine), a reading of 1 ohm or less. If higher ohms, you have a weak connection to ground on that wire. If that reading is < 1 ohm, by process of elimination, you have a discontinuity on the 12V wire. For any wire discontinuity, look for a crack in the wire jacketing. The copper inside corrodes and weakens once the jacketing is cracked. I've had to replace several lo-beam wire harnesses over the years, and recommend using only Volvo parts. Consider yourself lucky if this is your first headlight harness issue in 45 years!
2013 XC70 T6 Flamenco Red (hers)
2015 XC70 T6 Seashell Metallic (his)
past: 13 Volvos going back to '74 242 sedan
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