How do you reset the check engine light on a 2002 Trailblazer?

By having the problem that exists with the emissions controls repaired. Check to see if the gas cap was off or not on properly. If that's the case, the light will go automatically in 2 to 3 days. If not, then you have a bigger problem that would require mechanical help with diagnostic equipment. Any dealer or repair shop should be able to tell what's up.