The usual way to reset the Check Engine Light is to pull a fuse -- typically labeled "Back-up." Sometimes the ECU will set the CEL, but then the problem will go away. But once it's set, the CEL may stay on even though the problem isn't there anymore. If you re-set the CEL (pull the Back-up fuse), and the problem is no longer present, you're all set. The CEL will stay off.

But if there's still a "definite" problem, the CEL will get set AGAIN, after you re-set it. In that case, like Oofrost said, you gotta find and fix what's wrong. O2 sensor? EGR valve? Check the codes -- they'll point you in the right direction.

Also, like Oofrost said, 800 rpm sounds normal, maybe a TEENSY tad high, for a warmed up engine. Close enough, anyway. What's the problem there?