The stereo will simply say CODE on the screen. punch in the correct 5 digit number, and it beeps and you are back in business!

On some vehicles, the CEL could be as simple as a loose gas cap- again emmissions related. Worth a drive to Autozone to not only have code read, but they can Erase the codes without you having to disconnect battery, and reset your factory radio.

On second thought, the code erasing might only be on OBDII vehicles.