Ok I have had this problem for the past few months. I have a 20005 MCS with around 95k miles. I replaced the gas cap about a month and a half ago, for the 1st couple of weeks it was great, no light. But now the light turns on at least once, every week. I have noticed the light turns on when I get a full tank of gas and turns off when I'm low on gas.

Did I get a faulty gas cap?
I've read it could be the charcoal canister...where is that?
Then I've read some valve...where is that?

Does anybody have the diagram for the evap system?

Any help would be greatly appreciated!

 

Me too. My light has been on for months, but turned off yesterday after I gassed up. Shrug.

There is a valve at the front right side of engine. Mine looked good so that wasn't it.
There is the charcoal canister in the rear right side in front of rear wheel. There is also the Leak Diagnostic Pump attached to the canister that commonly starts its own leak. The sole purpose in life of this LDP is to turn on your CEL. You can source a new one for about $120 from Sewell.
You can make your own home brew smoke machine to find the leak or you can guess and buy a bunch of expensive parts. I am going to swap the LDP and if that doesn't do it I will go to a shop with a smoke machine.