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.

Disconnect connector C9 of the combination meter, connector A12 or A13 of the A/C ECU and connector E4 of the ECM. See Fig 1.

Check continuity between connector A12 terminal 1 (MPX-) of the A/C ECU and connector E4 terminal 21 (MPX1) of the ECM. See CONNECTOR IDENTIFICATION .
Check continuity between connector C9 terminal 21 (MPX-) of the combination meter and connector A13 terminal 10 (MPX+) of the A/C ECU.

If there is continuity in the wiring harness of both (A) and (B), (A) or either (B), replace the faulty A/C ECU located in the A/C control with radio receiver assembly. See INSTRUMENT PANEL article in ACCESSORIES & EQUIPMENT. 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 BODY COMPUTER in SYSTEM WIRING DIAGRAMS article.

Fig 1: Checking A/C ECU Communication Bus Circuit