2001 Malibu P1404 code

My 2001 Malibu has 15,000 miles (new engine dropped in at 13,000) in June 2005. Now my check engine light comes on. Took to my local mechanic. He cleaned the fuel system and cleaned up the part, put it back into the car, reset the computer and let the car run for 20 minutes. No light. I drive away and the light goes back on. P1404 is the code that comes up when running the scan. Does anyone have any idea what part would be making that code to light up? Where to I find the faulty part? My mechanic told me to take it back to the dealer to see if it would be covered under the 12,000/12 month engine warranty. I seriously doubt that would be covered under the warranty, but you never know... Thanks in advance.