if the cel is on constantly, but you have a normalized idle, it could be that the computer just adjusted to the problem and will try to substitute an idle speed conducive to it running somewhat properly. i'd unplug the battery and see what it does sans memory and see what problem is being covered up.

ps. i only say this because i know it from experience. if the ICV hose is kinked, the car will adapt a high idle (but steady and seemingly normal) of about 800-900-1000 rpm after about 30 mins or so of runtime. it could have adapted to whatever problem you have.