The 1-Series model uses a total of 4 O2 sensors. Two before the catalyst and two after the catalyst.Select O2 sensor location from the menu. Made by Bosch. Sold each. Part# 11 78 7 545 074 Post Cat in Front Manifold. Cyl 1-3 (560mm length) Part# 11 78 7 545 075 Post Cat in Rear Manifold. Cyl 4-6 (490mm length)
