98 Old Aurora idles 500 over while driving? Emission Control?

ok so my father has a 98 oldsmobile aurora we recently did a engine swap on Now it ran fine after then the service engine comes on a couple months down the road and it idles 500 over in gear instead of 1.5 at 60 its 2, now i hooked up my computer(will get u codes tmo) and all i got was emission control failure? other then that the car runs perfect. ne suggestions on what i could do too get this beauty savin gas like it used too