The front oxygen sensor is to detect the air- ratio of the engine in different working conditions. The rear oxygen sensor is mainly to detect whether the three-way catalytic converter works normally. 100% test before shipment. The reduction of harmful emissions. The increasing of economy. The improvement of engine performance. Prevent premature failure of the catalytic converter Specification: number:11787513962 11781433940 Replacement: Direct Plug-In Replacement Condition: 100% New Type:Oxygen Sensor Fitment Type:Direct Replacement Number of Pieces:4 Pls make sure that the Number:11787513962 11781433940 and the item picture fit your car before purchase 100% compatible Fitment: 2000 323Ci 2.5L 1999-2000 323i 2.5L 2001-2003 325Ci 2.5L 2001-2003 325i 2.5L 2001-2005 325xi 2.5L 2000 328Ci 2.8L 1999-2000 328i 2.8L 2001-2003 330Ci 3.0L 2001-2003 330i 3.0L 2001-2005 330xi 3.0L 2001-2003 525i 2.5L 1999-2000 528i 2.8L 2001-2003 530i 3.0L 2004-2005 X3 2.5L 2001-2006 X5 3.0L 1999-2002 Z3 2.5L 1998-2000 Z3 2.8L 2001-2002 Z3 3.0L 2003-2005 Z4 2.5L 2003-2005 Z4 3.0L 2003-2005 Land Range 4.4L Color: As shown Material: ABS+Metal Package Contents: 4*O2 Sensor Only the above package content, other products are not included. Note: Light reflection and different displays may cause the color of the item in the picture a little different from the real thing. The measurement allowed error is +/- 1-3cm.