Service Engine Soon (2001 Blazer)

The SES light came on at 35600 miles. My Dad bought a Creader and we plugged it in and a code came up 0410 - secondary air intake system failure. The computer then reset itself, but the SES light came back on after 150 miles. The Blazer is not running rough and does not have a rough idle, but the mileage is not so good (14/18) as opposed to what it used to get(16/21). The air filter is relatively clean. There are a couple of sensing elements right after the air filter assembly. Could one of these be the culprit? Has anyone changed any of these components? I don't want to have to spend the $85-$110 diagnosis fee at the "friendly" Chevrolet dealer if I can change the part(s) myself.

Thanks!!!

I think this may help
http://www.automotiveforums.com/vbulletin/showthread.php?t=368290

Also do a tuneup.