A fuel-injected engine uses many different sensors to determine how to make your engine run properly. The 1987 and newer GM trucks use one or more oxygen sensors located along the exhaust. They're not difficult to replace, but they can take some elbow grease and some basic safety equipment to do properly.
First, make sure the vehicle is turned off, has cooled down for a few hours and the emergency brake is set. Using a jack, lift the front of the truck so that the front tires are off the ground. A good jack point is the front crossmember, or alternatively, the sides of the frame below the doors.
With the truck properly supported, slide underneath the vehicle and look for the O2 sensor. Depending on the year of the truck, there will be just one sensor located on the passenger side near the transmission, or several sensors on the later models, located both before and after the catalytic converters. They're easy to spot, as they stick out about 2 inches from the exhaust tubing and have two wires coming out from the body.
Unplug the harness that connects the O2 sensor to the stock wiring. If the clip that holds the harness together is difficult to disconnect, use a small flathead screwdriver to pop it free.
Using an O2 sensor socket or an appropriately sized wrench, loosen and remove the old O2 sensor. It unthreads like any other bolt would, going to the left to loosen. Should the sensor be stuck in the threads, loosen them with some WD-40 or similar product. Should the sensor still not come loose, the bung can be cut out of the exhaust, but it will have to be welded back into place by a professional, and should only be done as a last resort.
Before installing the replacement O2 sensor, put some anti-seize compound around the threaded area of the sensor. This will help you the next time you have to replace the sensor, as the heat of the exhaust won't cause the threads to seize up on you.
To reinstall, thread the O2 sensor into the bung on the exhaust, reconnect the wiring and lower the truck off of the jackstands.
