code P0171?

can this code be caused by installing a new idle air control valve? i thought the iac just controlled the idle . i had a code p0306 misfire on cylinder six and found out the morons that worked on my 98 stratus had the 2 front spark plugs only finger tight,this caused the actual plug wire to pop up an inch off the spark plug and when i pulled out the plug wire for the #6 plug it was filled with gas. i then pulled the upper intake manifold and ended up finding the back 3 plugs were loose too. i torqued all of them and put the intake manifold back on,using the same gasket, it looked brand new,they just changed it. now i have this new code i didn't have before,p0171. they put in a new downstream O2 sensor but the 2 in the exhaust manifold are original,but no codes for any problems with them? does this code lean toward a vacuum leak of some kind? thanks for any info Randy