Illumination of the Check Engine Light can be caused by a loose or worn gas cap.Google+
Car Problem Reports
Hummer H2 Check Engine Light Due to Gas Cap Issue
Hummer H2 Problem
Average Mileage: 94,000 mi (30,000 mi - 124,000 mi)
Visitor, 2003 Hummer H2, 102,000 mi
Check engine light went on after awhile and not only was realized that putting in gas and attendant did not seal correcty.
Visitor, 2003 Hummer H2, 120,000 mi
shop said to get a new gas cap. mine cap cover just snaped off one day. so i got out the super glue. but now the cap will not click when i try to tighten it. so know i will buy a new one.
Visitor, 2004 Hummer H2, 124,000 mi
this is a very common problem replaced 2 gas caps due to engine light also 2 times on my 2500 series