How to Replace the Oxygen Sensor on a Blazerby Jody L. Campbell
Oxygen sensors plug into the exhaust system of most vehicles. Vehicles manufactured after 1999 have multiple oxygen sensors plugged into the exhaust system. These sensors monitor the fuel to air ratio and determine the amount of harmful emissions going through the exhaust system and send this information to the main sensor, closest to the engine head. Then the main sensor sends the information to the vehicle's computer. Perhaps the "check engine" light flicked on in your Blazer. The town mechanic scanned the code and told you it was the oxygen sensor. Since the replacement sensors are generally more than $100 to buy over the counter, there's no sense paying the mechanic labor charges to put it in provided you have some limited mechanical skills and a few tools.
Determine which oxygen sensor needs to be replaced. This can be done with a code reader and there are many repair shops that will check your codes for no charge. The code reader should indicate which oxygen sensor is giving the trouble code. Determining which sensor is bad will be the most challenging part of this job. On Blazers manufactured after 1999, there will be multiple banks and sensors. The bank closest to cylinder one is bank one, so finding where the number one spark plug in the engine could help you determine which side. The higher the number the sensor, there further back in the exhaust system it is placed. For example Bank One/Sensor One would be the closest sensor to the manifold on the cylinder one side. After the front Y-pipe on the exhaust system for the Blazer, it channels into a single exhaust before it streams through the catalytic converter. Some parts stores will have code readers and if you buy the oxygen sensor from them, then they will have a vested interest in helping you find out the information you need and sell you the proper sensor for replacement. Also, check the sensor before you leave to be sure there is a small amount of anti-seize compound on the threads. If not, purchase a small tube of it at the parts store as well. Most quality sensors come with there perfect amount already coated on the threads of the new sensor.
Raise the front of the vehicle on a flat, level surface. It is recommended to use car ramps for this, but you could use a floor jack and two jack stands to elevate the front. Just be sure the vehicle is secure and safe because you're going to be underneath it. Place the Blazer in park or gear and apply the parking brake. Turn off the ignition and remove the keys so no one turns it on by accident while you're underneath. Place a wheel chock behind one rear tire.
Crawl under the vehicle keeping the remaining hand tools handy so you can reach them as needed, including the replacement sensor. and examine the exhaust system. Use a creeper if you have one. Once you've found the sensor you're looking for, follow the wire coming out of it and trace it with your hands to feel where the plug is for it. It won't be more than 8 or 9 inches away. Look at the new sensor to determine the length. Once you've located the plug, use the small straight edged screwdriver to press in the lock and at the same time, pull the sensor plug apart.
Spray a small amount of WD-40 on the threads of the sensor where screws into the exhaust system. Not too much because if it doesn't feel like it wants to loosen, you may have to heat it up with the hand torch a little bit and WD-40 is obviously flammable. The key here is not to damage the threads on the exhaust pipe. If that occurs, you will not be able to rethread the new sensor in tightly. Try loosening the sensor using the 22 millimeter wrench or the oxygen sensor socket and a ratchet. Give it a good tug, but do not force it off. Usually, if its going to come off, a good tug will loosen it and you'll be able to unscrew it by hand almost immediately. If it does not, apply a little heat with the hand torch to the exhaust pipe around the circumference of the sensor. Do not heat the sensor and the pipe. That would swell both metallic components and you'd be defeating the purpose. You want to make the pipe swell and loosen the sensor. Just a little heat should be enough. Allow to cool once you've broken the sensor loose if you had to use the torch.
Remove the sensor and compare it to the new one. If the new sensor did not have a small amount of anti-seize compound on the threads, apply some now. Just a small amount and only on the threads. Do not get any on the sensor head. Screw into the sensor hole and tighten with the hand wrench. You want it tight, but as you've noticed, the sensor only have 4 to 5 threads, so you do not want to over-tighten and strip the threads or you're going to have to buy another sensor.
Plug the sensor in until you hear and feel it click. There's only one way it will fit into the plug. Remove the tools and lower the vehicle. The check engine light may take a few days and a few cycles to reset. It is recommended that you go back to the parts store and have them reset the check engine code for you and then run the Blazer under normal conditions for a few monitor cycles (may take up to 70 miles or so) to see if it comes back on. You can also try disconnected the battery, but this will erase other data stored in the computer.