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According to Alldata v10.40:
2001 Lexus IS 300 L6-3.0L (2JZ-GE) Copyright © 2011, ALLDATA 10.40 Page 1 Oxygen Sensor: Technical Service Bulletins Engine Controls - Bank 1/Bank 2 O2 Sensor Identification ENGINE EG015-07 Title: ENGINE BANK 1 AND BANK 2 A/F AND 02 IDENTIFICATION Models: '90 -'08 Applicable Models June 28, 2007 TSIB UPDATE NOTICE: The information contained in this TSIB supercedes TSIB No. EG010-04. TSIB No. EG010-04 is now obsolete and should be discarded. Introduction This service bulletin provides information on the proper identification of engine bank 1 and engine bank 2 for correct A/F sensor and oxygen sensor replacement. This bulletin contains information that identifies engine bank 1 and engine bank 2 on the following engines: 2GR-FE, 2GR-FSE, 4GR-FSE, 2JZ-GE, 1MZ-FE, 3MZ-FE, 1UR-FSE, 3UR-FE, 1UZ-FE, 2UZ-FE, and 3UZ-FE. ^ Bank 1 (B1) refers to the bank that includes cylinder No. 1. ^ Bank 2 (B2) refers to the bank opposite bank 1. ^ Sensor 1 (S1) refers to the sensor that is located before the catalytic converters. ^ Sensor 2 (S2) refers to the sensor that is located after the catalytic converters.