2004 Chevrolet Trailblazer L6 4.2L Downstream O2 Oxygen Sensor TRQ OSA61247

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In this video, we're going to replace the downstream oxygen sensor. It's the sensor that's after the catalytic converter in the exhaust. You'll need a jack and jack stands; you are going to want to raise up the front of the car and support it. After that, you'll just need a 7/8 inch, open end wrench to do this repair.

Here's a little perspective shot going from the back of the car. You want to raise and support your vehicle with jack stands and you can go right up and the oxygen sensor is right there. The connector is actually, right over here. It's a little hard to film but there's a blue safety lanyard that you want to pull out of the connector, first. Then there's a tab to pull back on and pull your connector apart. Now, we're going to use a 7/8 inch, open end wrench to put up in there. Get the right angle on it. You should be able to break the oxygen sensor free. Actually, once it comes free, they actually, usually come out fairly easily.

Put the new oxygen sensor in place and you can tell we're not actually not putting a new one in this car. There's nothing wrong with the original one, so we're usually not ones to waste a new part where a car doesn't need it. We just wanted to do this to show you, but you put it back in. You just have to work with the harness a little bit and get it finger tight as well as you can. Put your 7/8 inch wrench back on there and tighten it up. Now, you're probably going to find that aside from jacking up and putting the car on jack stands, that the most interesting part of this repair is just trying to get the connector, reconnected and especially since we're trying to film with one hand and do the connection with the other hand, makes it a little more difficult, but reconnect the connector. Put it back into its clip and also put that blue safety clip back in.

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In this video I have a couple of oxygen sensors with me. While there are some aesthetic differences here, the wire lengths are a little different, the actual body and tip of the sensor are a little different, the threads and gasket are the same, and on the other end the connections are actually the same. I can plug these into either position on our vehicle. But if I had to ask you which one's upstream and which one's downstream, you probably can't tell just from looking at them. That's true of most oxygen sensors. The way you determine whether or not an O2 sensor is upstream or downstream is all done visually.

Your oxygen sensor position can be determined relative to your catalytic convertor. As the name suggests, upstream sensors will be located upstream or ahead of, in front of, above, closer to the engine than your catalytic convertors. This means that they see the exhaust gasses first. They then go through the catalytic convertor and to the downstream oxygen sensor, which will either be located in the catalytic convertor like our vehicle here, or after the catalytic convertor to monitor the gasses coming out of the cat and make sure that it's being efficient relative to the readings of the upstream O2 sensor.

The other determination you need to make is bank one versus bank two. Determining which bank is which can be done by looking at a picture of your vehicle's firing order. Whichever bank contains cylinder one is going to be bank one, and the other one, regardless of whether or not cylinder two is in it, is going to be bank two by default. These banks are true of V engines like V8s, V6s, and flat engines like Porsches and Subarus that have individual banks.

On inline four-cylinder engines, as long as they have one exhaust manifold like a Jeep four-liter or a four-cylinder Honda engine, you'll only have one bank with an upstream and downstream O2 sensor. There will be no bank two on the engine.

On our vehicle here, this is the passenger side, which we've already determined from the firing order is bank one. We have our upstream sensor here closest to the engine ahead of the cat. Then we have our downstream or sensor two here inside of the catalytic convertor. This could also be located somewhere after the cat. Here I have my two sensors for the vehicle. While they are aesthetically different, we can now tell this is my upstream sensor because it looks just like the upstream sensor I have up there. This is my downstream sensor. Now that we know which one's which, we can order the right part, correct the problem, and fix you up right.

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