'01 Infinity I30's have 4 O2 sensors. Two in Bank one and two in Bank two. They are located on the exhaust system. On the front of your engine you will see four harnesses, blue, red, white and black. All but the blue wire runs under the engine to the exhaust system. Bank 1 is the rear of the exhaust and Bank 2 is the front of the exhaust (by the manifold). Black and white go to Bank 1 and blue and white go to Bank 2. Go to infinitihelp/Ownership/common.com and there is a diagram of the exhaust system. Your most likely looking for the black harness/wire.