2004 GMC Envoy Engine Light On

For about 3 weeks my Envoy has the Service Light
service soon. I bought a new GMC gas cap. The
light went out, but has come back on now. At 41,000 I had to replace the gas tank to take care
of the problem. License are due and I need to run
my car for testing. And It willl not pass with this light
on.