It depends on what e36 it is. The 325i you and I have, has one. It's right in the middle of the whole car, where the two exhaust pipes come together just for the sensor and then split again. I tried changing mine by myself but there isn't enough space. I got tired of it and went to a mechanic. The guy couldn't get to the sensor even with the special tool, he had to disconnect the exhaust from the headers to get to the sensor. Good thing I didn't bother getting the special tool.