RepairPal Certified shops can diagnose and repair your 2001 Ford Explorer Sport Trac. They provide transparent pricing with a minimum 12-month or 12,000-mile warranty on repairs and services.
This could be due to any number of problems that your engine computer is detecting within your emission control system. RepairPal has an article that talks about what could be causing it and what to do next: http://repairpal.com/check-engine-light