It doesn't tell you pressure or which tire is low. Just a simple light will come on to tell you that one or more don't meet the threshold of pressure.
http://www.nhtsa.gov/cars/rules/rulings/tirepresfinal/index.html
"This document establishes two compliance options for the short-term, for the period between November 1, 2003, and October 31, 2006. Under the first compliance option, a vehicle''s TPMS must warn the driver when the pressure in any single tire or in each tire in any combination of tires, up to a total of four tires, has fallen to 25 percent or more below the vehicle manufacturer''s recommended cold inflation pressure for the tires, or a minimum level of pressure specified in the standard, whichever pressure is higher. Under the second compliance option, a vehicle''s TPMS must warn the driver when the pressure in any single tire has fallen to 30 percent or more below the vehicle manufacturer''s recommended cold inflation pressure for the tires, or a minimum level of pressure specified in the standard, whichever pressure is higher. Compliance with the options would be phased in during that period by increasing percentages of production."
"The second part of this final rule will be issued by March 1, 2005, and will establish performance requirements for the long-term, i.e., for the period beginning on November 1, 2006. In the meantime, the agency will leave the rulemaking docket open for the submission of new data and analyses concerning the performance of TPMSs. The agency also will conduct a study comparing the tire pressures of vehicles without any TPMS to the pressures of vehicles with TPMSs, especially TPMSs that do not comply with the four-tire, 25 percent compliance option."
