First of all, thank you in advance for any help that you may be able to provide.

I have a 2006 Cayenne S. I am the second owner and have owned this car for about 10 years. It has always been serviced at the recommended intervals. Approx 142k miles.

The 4 wheel drive fault message keeps coming on. I took it in and was told (local mechanic) that the transfer case motor was bad. I also had one one of the wheels locking up and found out it was the front passenger side wheel speed sensor (which I had replaced).

What has already been done:

* I have a brand new battery. I only have one in my car. There is a subwoofer under my spare in the back. I have double checked to make sure.
* I replaced the transfer case motor.
* I also went ahead and replaced all the wheel speed sensors so I didn't have another one go bad and add to the frustration.

I am getting ready to replace the transfer case oil in the next day or so as I have read that having old oil can also cause some error messages.

Is there anything I could check to narrow this down?