Possible causes

– Controller Area Network (CAN) communication harness is open or shorted
– Faulty Engine Control Module (ECM)
– Poor harness ground connection.

Tech notes

There is a factory service bulletin for the following Infiniti models:2002-2004 Infiniti I35
2002-2006 Infiniti Q45
2004-2006 Infiniti QX56
2003-2006 Infiniti FX
2003-2006 Infiniti G35
2003-2004 Infiniti M45
2006 Infiniti M
Infiniti Service Bulletin Code U1000
Service bulletin suggest cleaning engine and body harness ground connection to repair problem.

There is another Infiniti service bulletin with tips to diagnose the U1000 code:
Infiniti Service Bulletin Code U1000 Diagnostic Tips and Guidelines

When is the code detected?

When ECM is not transmitting or receiving CAN communication signal of OBD (emission related diagnosis) for 2 seconds or more.

Symptoms

– Engine Light ON (or Service Engine Soon Warning Light)

U1000 INFINITI Description

Controller Area Network (CAN) is a serial communication line for real time application. It is an on-vehicle multiplex communication line with high data communication speed and excellent error detection ability. Many electronic control units are equipped onto a vehicle, and each control unit shares information and links with other control units during operation (not independent). In CAN communication, control units are connected with 2 communication lines (CAN H line, CAN L line) allowing a high rate of information transmission with less wiring.
Each control unit transmits/receives data but selectively reads required data only.

INFINITI Technical Service Bulletins for U1000 code