The 2004 GMC Sierra truck has several O2 or oxygen sensors in the system. On the V8 engine equipped trucks, there is one in each exhaust pipe in front of the catalytic converter as well as one on the down pipe coming from the exhaust manifold. When determining if you need to change an O2 sensor, you need to note which one of the sensors is faulty so you can replace the right one. This information will be stored in the truck's computer, so have it scanned before you change the sensors. Most auto parts stores will do this for you free of charge.
Position a jack under the front suspension of the truck and raise it off the ground. Position a set of jack stands under the front suspension or frame rails to support the truck, then lower the jack and remove it from under the truck.
Locate the O2 sensor you intend to change. If it is the upstream sensor, find it under the engine in the down tube that runs from the exhaust manifold to the catalytic converter. If it is a downstream sensor, it is in the exhaust pipe just in front of the catalytic converter. On dual exhaust trucks, there are two downstream sensors on in each pipe.
Locate the wiring harness pigtail coming out of the top of the O2 sensor. Follow the wires up to the electrical connector at the vehicle wiring harness. Disconnect the connector by pushing down the locking tab on the connector and pulling the two halves of the connector apart.
Place an O2 sensor socket on the O2 sensor in the exhaust pipe. If you do not have one, you can buy an O2 sensor socket at most auto parts stores. Using a 3/8-inch drive ratchet, turn the socket counterclockwise, loosening the sensor. Lift the old sensor out of the pipe and discard it.
Thread the new O2 sensor into the hole in the pipe by hand to avoid stripping or cross threading fit. Tighten it with the O2 sensor socket and ratchet, then run the wiring harness pigtail up to the connector on the truck. Push the connector on the O2 sensor into the wiring harness connector, making sure the locking tab engages. Repeat this process for each O2 sensor that you need to change.
Position your jack under the front of the truck again and lift the truck off the jack stands. Remove the stands from under the truck and carefully lower the truck to the ground.
