The low tire pressure warning light in my Subaru came on yesterday; we think it’s because of the dip in temperature. This was the second time I’ve seen the light. The first time my tire had picked up a nail. So I finally decided to look up how the sensor works, especially since I was complaining that it couldn’t tell me which tire was low. The Subaru “tire pressure monitoring system (TPMS)” consists of a sensor and transmitter in each tire, connected to the valve stem, and a receiver under the driver’s seat. The sensor batteries are expected to last 10 years or 100,000 miles. If the receiver doesn’t receive four signals, the warning light will flash, and although each sensor does have a separate ID code, there’s no method to determine where that tire actually is. All cars sold in the U.S. now must have TPMS and fancier cars do let you know which wheel is low.