headlites work on highbeam but not on low beam.
Have you physically checked the low beam bulbs to make sure they’re not burnt out (assuming you have separate bulbs for high/low)?
If the bulbs are good check for voltage at their sockets - if none then check the circuit diagram and start working your way through the components - fuse(s), low beam relay, headlight switch, possibly a multi-function relay, etc. Sometimes circuit boards develop bad solder joints or failed traces (common with some bulb monitor relays that affect multiple lighting systems). Pay attention to return paths (grounds).
Take at a look at this post in the subaruoutback.org Generation 3 forum: