First he installed brand new Flosser D2S bulbs. That didn't fix the problem.

Then we installed a "used but working" Hella ballast I bought from EBay on the drivers side. That didn't fix the problem. Then we swapped the ballasts, so the one originally on the pax side is now on the drivers side.

In order to isolate whether it was a problem within the headlight assembly, or something further upstream, we removed both assemblies and temporarily hooked them up on the wrong side. Then they both worked.

Then we hooked them back up on the correct side, but the retaining cages were removed, and we only had them held in place with one screw. They both worked.

Then we removed the headlight assemblies, reinstalled the retaining cages, and put it all back together with all the screws... and we are back to square one - Driver side low beam doesn't work.

Will try the software suggestion. Thanks & speedy recovery to you.

Is there any point trying to disconnect battery, wait, and reconnect? I see that often suggested in other threads.