15.1 is the high and it will vary between 12.5-15.1, it's still in the normal range. Go for a drive on the highway for about 20 miles and see what it does. It should start to drop after you have been on the freeway for a while. If you were working on the car for a while and you were having some drain from a device like radio or lights it will start out high then eventually fall after a while. It could also be in reconditioning mode where it raises the voltage to that level for a while.
Not real sure why you are getting a Charging System Failure... Is the current sensor on the Negative battery cable there and are all the negative wires going through it? There are 3 on a GTP and I am not sure on a GXP, but I would have to assume it's the same.
This is a normal chart for charging that I logged a while ago, from start up through a few hours of driving. The top is the Control Module voltage and the bottom is the voltage measured at the Diagnostic Port.
Jeff
