Has the "check engine" light been on at all while the engine is running? If not, then there might not be a diagnostic code to read, anyway. If no "check engine" light then there might not be any faulty sensors or actuators (i.e. nothing for the ECU to report)... which probably means a dirty MAF sensor, or maybe just a clogged air filter. If I were you, I would try cleaning the MAF sensor first with Isopropyl Alcohol - commonly sold as electrical contact cleaner. Give it a good soaking a few times, but don't touch the delicate electronic bits with anything. If that doesn't solve it, try a new air cleaner. If that doesn't solve it, try resetting the ECU.
As suggested before, check out the Error code PDF Error Code PDF Post. Even without knowing an error code, the listed symptoms might give you a clue as to a faulty sensor.
Are there any other symptoms? Is the car hard to start after stalling?
This MIGHT help...: OBD Diagnosis
Cheers