OK, so driving to work Friday morning low beam headlights work fine. However, driving home from work on Friday eve I get the "low beam bulb failure" message on the dash. Once on back roads, it seems very dark. So I turn off the fog lights (which I run pretty much all the time). Wham - I have no headlights! Low beams both failed. But, at least I still had high beams and fog lights.

So, immediately I suspect some sort of electrical "issue". Saturday I google for this problem and get all sorts of hits like this one:
http://www.matthewsvolvosite.com/for...?p=51613#51613

I spend most of my Saturday on my back with my head in the driver's footwell looking at the CEM. I pulled and cleaned shunts. I pulled and cleaned relays. After no luck, I tried swapping a known good shunt (right turn signal) with the suspect one. Then I tried swapping a known good relay (hi beam) with the suspect one. Still, no luck. Huh! Frustrating.

Then, on the absolute outside chance/coincidence that BOTH headlight bulbs burned out simultaneously, I check the bulbs. Sure enough, both bulbs failed simultaneously (or within 10 minutes of each other).

No idea how that could have happened but the guy at the friendly local auto parts store says he's seen it before. One goes and it somehow overloads the remaining one.

So, just goes to show not to over complicate things. The simplest answer is usually the correct one.

In the end, I love trouble shooting things like this but, get frustrated when I can't solve it. So, glad I finally solved it. And very glad it didn't cost me $1,000 for a new CEM from the dealer. Just disappointed I took the longest possible route to getting there.

Bob

 

Always start with the easiest things when it comes to diagnosis. K.I.S.S should always be in your head