Possible Causes

WARNING: This page is about a different car, the 2003 Dodge Dakota. However, it is still accessible from the selected car via links, so may be relevant.

Charging voltage below 15.1 volts.
Battery positive circuit high resistance.
Case ground high resistance.
Generator operation.
(Z13) Gen ground circuit open.
(K20) Gen field control circuit shorted to battery voltage.
(K20) Gen field control circuit open.
(K20) Gen field control circuit shorted to ground.
PCM.