It's usually a sensor or a misfire. After 3-5 start cycles, the light goes off when the problem goes away. I would suggest a tuneup if it hasn't been done recently, O2 sensor. I had a loose vacuum hose trigger that as well as sparkplug wire crossfire.
This may sound stupid, but did you just fill up the tank or switch brands of gas? A tank of bad gas will set off the light, but your car will run fine. Also, if you don't click the gas cap at least three clicks, the light can come on. The OBD II computer is very sensitive. I added some stp gas treatment when I got a bad tank of gas, and my light when off. If that doesn't work, probably a bad O2 sensor. Good Luck
it can also mean you need to service the car. the puter's do this cause there are part's in the emision system that need to ba changed at various milage mark's. normaly this happen's at around 50,000 mile's. i do not know if G.M is still programing this into the ECU. i know the jap's are still doing it. and ford too.
also if the mechanic that serviced it did not clear certain code's you get the light, even if you have done the srvice on the car.
[This message has been edited by ilbl8 (edited 04-05-2001).]