Back in April I purchased 4 new tires and wheels from Tire Rack. They came mounted and balanced with new TPMS sensors and all worked well until about 2 weeks ago. My low tire light came on. Checked all of the tires and they were good but I put a bit of air in them anyhow. The door sticker says to run at 37 psi, but I had good luck with the tires at 32 psi for several months so I don't think it was the slightly lower pressure that did it. I now have 35 psi in them but the light is still on so I'm guessing one of the sensors is bad, possibly the spare since it is still original. How can I tell which one is setting off the light? Thanks!