Illumination of the Check Engine Light can be caused by a loose or worn gas cap.Google+
Car Problem Reports
Chevrolet Avalanche 1500 Check Engine Light Due to Gas Cap Issue
Chevrolet Avalanche 1500 Problem
Average Mileage: 121,928 mi (73,000 mi - 189,000 mi)
Changed Gas Cap like Auto parts repair said to do and did not fix the problem.
Visitor, 2004 Chevrolet Avalanche 1500, 97,000 mi
I've had a continuing "check engine" light from a code indicating a vapor leak in the fuel system. After a couple of times resetting the light with a portable instrument (after clicking down on the gas cap), I changed the cap. That was in late February of 2012. Just last week, after getting fuel, the check engine light is on again.
Visitor, 2004 Chevrolet Avalanche 1500, 73,000 mi
check engine light and tighten full cap display
Visitor, 2004 Chevrolet Avalanche 1500, 189,000 mi
same issue replace cap still cjeck eng lighy
Visitor, 2006 Chevrolet Avalanche 1500, 125,500 mi
MIL light came on and OBDII code reader indicated an "Open EEC Circuit" condition. Problem cleared by replacing gas cap with a "new" lockable cap and resetting the MIL light.
Visitor, 2005 Chevrolet Avalanche 1500, 115,000 mi
faulty gas cap