After being stumped for some time about why only LEFT headlight not be working on 2001 Nissan Pathfinder (already checked bulb, fuses & relay), I took the car to my mechanic for repair. The problem turned out to be a "dirty" Daytime Light Control Module (DLCM) which is required in Canada. Oddly in the Pathfinder, the left/Driver-side headlight only gets to Ground through the DLCM and when that is defective, the left haedlight ceases to work. The right/passenger-side headlight does not go through the DLCM and wasn't affected by the problem (this has to do with different parallel vs serial circuit design). It seems the DLCMs in some cars require some resoldering, though in my case, a simple "cleaning" and reseating solved the problem, so my excellent Japanese mechanic (Kunio in Ottawa) saved me $280 by not having to replace the DLCM. Hopefully this explanation will be of help to others who may experience a similar problem ... Joe.