Here is what my Toyota dealer told me when I bought my USDM Toyota Camry. Only expensive cars have these sensors, such as Porsche and Mercedes. He said Toyota's system is activated by measuring tire RPM---the theory being that a flat tire will rotate more at the same speed compared to a fully inflated tire. Thats why he recommended to regularly check tire pressure as uneven tire pressure could also activate the system---and yes, it did once one very cold morning. Its been almost ten years, and I still have to verify (Google) to see if all he said was BS.