Diagnostic Procedure

WARNING: This page is about a different car, the 2003 Lexus IS 300 and 2002 Lexus IS 300. However, it is still accessible from the selected car via links, so may be relevant.

Check that the engine starts normally. With this inspection, the ECM can be diagnosed if it works normally or not. If the results are OK, go to the next step. If the results are not good, replace the faulty ECM. See ENGINE CONTROL MODULE (ECM) under REMOVAL & INSTALLATION.
Disconnect the connector of the body ECU, connector E4 of the ECM and connector A12 of the A/C ECU. See Fig 1.
1. Check the continuity between 12-pin connector terminal 7 (MPX1) of the body ECU and connector E4 terminal 20 (MPX2) of the ECM. See CONNECTOR IDENTIFICATION .
2. Check continuity between connector E4 terminal 21 (MPX1) of the ECM and connector A12 terminal 1 (MPX-) of the A/C ECU.
If there is continuity in the wiring harness of both (A) and (B), or either (A) or (B), replace the faulty ECM. See ENGINE CONTROL MODULE (ECM) under REMOVAL & INSTALLATION. If there is not continuity, repair or replace the faulty wiring harness. If there is an open in the wiring harness of either (A) or (B), repair it. See COMPUTER DATA LINES in SYSTEM WIRING DIAGRAMS article.

Fig 1: Inspecting ECM Communication Bus Circuits