1999 ford expidition won't start. Light on dash says theft. What to do?
PATS problem (passive anti-theft system). If you have another key try it. If you still have issues try it when there is no cell phone in the vehicle or on you. Sometimes (rare) a cell phone can interfere with the PATS keys.
Time for some reading http://www.aa1car.com/library/troubleshoot_antitheft_system.htm