engine light on after passing emissions

What is your question?

1999 mercury mountaineer. Weeks ago engine light came on. Filled gas tank and nee gas cap on. Light went out immediatly. 5 days ago it passed emissions test. Now light back on. Filled gas tank and tightened gas cap but light still on. I occassionally smell gas briefly in truck while driving sometimes. How long have you had this problem? a few weeks