hopping for some help

so the check engine light came on and i took it to a mechanic.... they said the light was indicating the vacuum system, and ran a smoke test to see if there were any leaks. The mechanic couldn't find anything that would have set off the check engine light so he turned off the light and said it was fine. Driving on the highway as the truck shifted in to passing gear the light came back on. Does anyone know what this problem could be ? I need to get it fixed so i can emission the truck and drive it again ...... 2000 tacoma 6cyl 4wheel drive


thanks

 

Buy a new gas cap.

 

Be nice to know just what code it was tripped