Check engine light?

Thanks I actually already fixed it and the light is off. The little gasket thing around the cap wasn't on right so I just put it back in place

'01 Mazda Protege; 2.0 5-spd. Manual; 114,000 Miles.

My wife started and warmed up the Mazda in our driveway after a foot of snow had fallen, so it idled for some time surrounded by snow. Thereafter, the Check Engine light came on. I started the car this morning and the Check Engine light remained on. Additionally, the cars RPM's was over 3,000+ while idling. Curiiously though, each time I engaged the clutch, the RPM's dropped to normal (900). After attaining normal operating tempature, and with the clutch disengaged, the engine then began to oscillate, with the RPM's going up to 2,000 - 2,500, then dropping back to 1,000 and then repeating this cycle over and over. Any help appreciated.