My left rear turn signal is inoperative. The fuse & bulbs are good. The 4 way flasher works as well. All other turn signals work. When I checked the socket with a meter, no voltage when left signal turned on. What can the problem be? Thanks

Check your Brake lights and see if they all work when you have the turn signal switch on.
I have both a 89 and 90 olds Cutlass supreme's and it the turn signal switch go's bad it will take out the brake lights (but not all of them) and the turn signal on one side. the reason I say it could be that is because I have replaced it in both cars with ones out of buicks that were in the junk yard and now they are fine