Take it to Autozone/Advanced and have them run an ARBST test (it's free). This will tell you if the alternator is putting out enough amperage, or has a bad diode, and also load test your battery as well.
Your ECU will rob power (shut down) accessories, when not supplied with enough power, to attempt to keep the engine running. I can't even count the number of times Saturn customers came in with a shift to D2, ABS, and other lights on, due to alternator/battery problems.