Section 779 (Multiplex Communication System - Diagnosis): Pre-Check: Notes

WARNING: This page is about a different car, the 2004 Lexus IS 300. However, it is still accessible from the selected car via links, so may be relevant.

DIAGNOSIS SYSTEM
1. As shown in Fig 1 , each ECU of this vehicle is connected by communication bus and it transmits is self-diagnosed by body ECU and it memorizes DTC when it detects communication stop to ECU or communication bus +B short or GND short. There is a possibility the Body ECU cannot self- diagnose accurately unless it doesn't work normal. So please note that the trouble- shooting of Body electrical system should be done after confirming if body ECU and open door indicator works normal by 2" BASIC INSPECTION" described later
  
  Fig 1: ECU Communication Transition Signal Diagram
  Courtesy of © TOYOTA, LICENSE AGREEMENT TMS1002
2. If DTC of ECU communication stop is output, there is a possibility of connector disconnection or 2 communication buses open. it will not become abnormal with only communication bus open
  
  Fig 2: Identifying Conditions For ECU Output Of DTC
  Courtesy of © TOYOTA, LICENSE AGREEMENT TMS1002
3. If 2 communication buses are open at the position as shown in Fig 3 DTC of ECU communication stop between those 2 buses is output
  
  Fig 3: Identifying Open Communication Buses (Preventing Communication With 3 ECUs)
  Courtesy of © TOYOTA, LICENSE AGREEMENT TMS1002
BASIC INSPECTION
Basic inspection is for checking body ECU normal operation