Re: unusual tpms sensor problem help!
Have you tried rotatiting the problematic tire/sensor to the front and/or swap the spare and put it in the rear left position?
If the problem follows the wheel, it is the sensor, if it stays where it is, I would assume it is the transponder. I believe there is a transponder for each wheel...but could be wrong.
I would also make sure the reset procedure for the TPMS system was performed correctly. I have no idea what it is (other than it is done with the StarScan), but when I have had replaced faulty sensors before, this took a couple of trips to the dealership to get right.