Changing the front oxygen sensor on a 1995-97 850

Oxygen sensors (also known as 02 sensors for short) are sensors placed inside the exhaust system of most cars made after 1985. Volvo also employed these sensors. The purpose of 02 sensors is to determine the ratio of oxygen to carbon monoxide from the vechicles emissions. reading the amount of oxygen makes the 02 sensor send information to the engine to make it inject more fuel (creating a "rich" combustion) or to make it inject less fuel (creating "a lean" combustion). A rich situation makes the engine burn more fuel than nessecary, reducing fuel economy. A lean situation burns too little fuel, increasing risk of engine damage and reducing power output. For most Volvo 850's, many 02 sensors are the original sensors from 15 to 18 years ago. If you feel the engine running strange or a "surging" sensation during acceleration, have a check engine light, or if by reading your engine codes with an OBDII code scanner you read a lean or rich condition, you may need to replace your O2 sensor.

Tools needed:

ramps or jack stands
needle nose pliers
3/8" socket wrench
10mm socket
12mm socket
21mm box end wrench
WD40 or equivilent penetrating oil.

(1.) Optional first step: Since 02 sensors are close to the asphalt and are subjected to rough, moist conditions, you may want to soak the 02 sensor in WD40 to loosen the rust. A rusty 02 sensors can increase the risk of rounding off the 02 sensor bolt, which makes extraction more difficult.

(2.) Drive the front of the car up unto ramps or use a jack and jack stands to prop up the front of the car.

(3.) Go underneath the car and unplug the O2 sensor, as shown below. It is located about 4' away from the front of the car.

In this picture, you can see that it is located near the exhaust manifold. You may need to remove the cross brace. I managed to remove my oxygen sensor without taking out the brace.

(4.) Use a pair of pliers and pull out the clip that holds the 02 sensor wire in place. It is plastic, and due to age and heat, may be brittle. Carefully remove it, or just buy a replacement clip.

(5.) Here is the 02 sensor. Use a 21mm box end wrench or a specialty 02 sensor flared wrench to remove it. If you have coated it in penetrating oil as stated before, it should come out easily. If not, be careful to not round off the edges of the hex head, or you may have to drill it out.

(6.)Here is the new O2 sensor installed. Installation is the reverse of removal.

(7.) Once the new 02 sensor is installed, use your scan tool to read the engine codes. If any codes are found, erase them. Now your done!