My girlfriend's sisters car is a 2002 Mazda Protege 2.0L with a 5 speed. The check engine light is on, and once it warms up, when she is sitting at a red light in neutral, the car idles very very goofy. It is around 800 rpm, then it just drops down to about 100-200 (almost stalls out) then bounces back up to around 800 and just keeps doing that until she drives off. No hesitation when she takes off, just this weird, "bouncing" idle. I don't know too much about newer cars, my car is an old-school Volkswagen Beetle and thats all I've ever worked on other than an occasional belt change in a newer car, so if it is something like a sensor or EGR valve whatever that is (my car doesn't have sensors or EGR valves) please go easy on me and let me know what it looks like, where it is and all that stuff. Thank you all so much for your help, I really appreciate it!!