I have a 2006 S80 and the left headlight is driving me crazy. Sometimes it is on and other times off with the message light on in the car that says low beam out. I know this sounds crazy and may be a huge coincidence but the light comes on in the daytime and goes off at night. I went to get the bulb replaced yesterday and the guy laughed at me because it was on. Last night I had to drive out on a dark country road and the light was off. When I got home I ordered a headlight from Amazon which I am now wondering if I will even need when it arrives. Today I went out and the light was on again all day. Tonight in the dark I ran over to the store and the light was off. This has been going on for two weeks or more. Am I losing my mind or is there some kind of sensor working in reverse?

 

Probably the low beam relay in the CEM is going bad or you have a loose connector into the CEM (central electronic module) Not sure where its on the S80s - google for any CEM posts or look for repair vids on youtube specific the 80s.

 

The relay would power both lights, not just one side. Check the connector at the bulb. Its common for them to melt.