Thanks, that thread was enlightening. I didn't think to Google. I first tried resetting the SMC on the logic board - no change. Upon raising the upper case, I noticed the left fan wasn't spinning at all, whereas the right fan was. I removed both fans, plugging each into its opposite connector on the logic board. Now the right fan doesn't spin but the left one does; and the hardware test reports: "4MOT/1/40000002: Left_Middle-Front" This would seem to eliminate the logic board so I'll go ahead and order a replacement left fan.