what is U1000 Controller Area Network Communications Circuit means?
Usually the problem is poor harness ground connection. The service bulletin suggest cleaning engine and body harness ground connection to repair problem. You can find a copy of the service bulletin here: http://helpforcars.net/obd_codes/u1000_nissan.html
It means there is a problem with the communication circuit between modules. And as tenspeed has put twice now there is a service bulletin that may help the concern.
Looking for a Used Altima in your area?
CarGurus has 82,542 nationwide Altima listings starting at $670.
Search Nissan Altima Questions
Nissan Altima Experts