1999 Chevy S-10 Poor Gas Mileage

Engine Performance problem
1999 Chevy S-10 6 cyl Four Wheel Drive Automatic 107 KM miles

I have even chipped my truck to try and get better gas mileage. Averaging 12 mpg's. New gas filter, air filter, plugs and wires. I got this code, (p1626), which is the Theft deterent system fuel enable circuit. Can this be my problem? It still hauls really good. My 95 S-10 used to average 20 mpg's in the city. Same motor!