I don't think the '03 had the serial bus communications and the speedometer readout diagnostic routines that current bikes have, so reading the codes will probably require a trip to a shop that has the proper scan tool. If you have access to a 2003 Electrical Diagnosis Manual, it should tell you if there is another method.
The fact that the check engine light only comes on for 8 seconds or less after startup and then goes out indicates there is a historical code that is not currently active. In other words, something set that code but the condition that set it is no longer a problem. Eventually, after about 50 engine run and stop cycles, the code will clear itself. If you want to clear it immediately, I'm pretty certain you can do that on the older models by eliminating battery power to the ECM for a few minutes. Pull the ECM fuse, or just disconnect the battery. If that eliminates the light coming on after startup, fine. If the light starts coming on again, don't clear the code and see your favorite mechanic.
Jerry