Replacing the O2 sensors is not only better for the environment, but it also helps improve your fuel economy. This simple guide will show you how to replace the O2 sensors in your XC90 Volvo and get you well on your way to saving money with better mpg.

Unplug the sensor, push in the top part and pull it out. The new sensor should come with anti-seize lubricant on it already, just slip it into the O2 socket and hand tighten it down. Make sure not to twist the wires around too much. Torque to 18-20 Nm per instructions. Now follow the wire down till you get to the new black clips. Push them into the appropriate holes where you cut the old clips out of. They require some force to push in, but they should click and stay firmly seated. Reconnect to the top array and make sure that the heat wrap is properly covering the new O2 sensor wires. I added a zip tie to keep it in place. Red O2 sensor is the downstream one.

Some time after 1996 a message came out about the Mass Air Flow connector on the turbo models. In time and because of the shortness of the connector from the MAF connection and the wiring harness, a resistance can build up on the connection. In time due to vibration, the connector can become chaffed, causing a bad connection.

To start with, the above photo shows the old sensor (left) and the new sensor (right). The new sensor has an added wiring harness/connector. This extra section of wiring is to help keep reference wires from clogging up with various engine deposits over time. When purchasing a new sensor be sure to clarify "turbo" or 'Non-turbo". When you get the new sensor make sure you have the extra wiring harness (it should be connected), the front sensors have a black connector and the rear sensors have a light gray connector. The remaining pictures are from a "non-turbo", which make it easier to take pictures of, however the ""turbo" components are in the same place, just a bit harder to get at because of the turbo piping.

Complete electrical diagrams for the V40 and S40 Volvo.  Engine to brakes lights are covered in these schematics.