w126 02 sensor replacement/installation
 

I'd been meaning to do something about the fluctuating idle when coming to a stop on my 1988 300 SEL for some time. When the CEL came on briefly a month or so ago and indicated oxygen sensor, I decided the time was now. I just finished the job. Pretty simple really, but I hadn't found a true write-up on the the forum so I thought I'd put down my thoughts. My idle is much smoother now, and I am very pleased with the results.

I have read that you can buy universal or oxygen sensors for other cars that are cheaper and splice the wires in. I didn't really feel like doing that. The bosch o2 sensor was only 80 bucks so I figured it was worth it to make the job easier. Supposedly these need to be replaced every 60,000 miles. My car has 112,00 and I don't think it had ever been changed.

The following is for a w126, it seems similar on other cars, but I'd check first.

1. Lift up carpet on front passenger side. You'll see the black plastic panel in the picture below. Remove the 10mm nut holding it in place and take out plastic panel.

2. In the next picture you see a set of wires in the bottom left side of passenger footwell. Disconnect both sets of wires. Follow them up to bottom edge of left sidewall where you'll see a black rubber plug. Push this plug out followed by the wires until they are entirely through the hole.

3. I found it easier to access the oxygen sensor from the driver side of car. I just slid under the car, didn't bother jacking it up. Right about where the two sets of exhaust tubing join you will see a wire leading into the top of the Y like in the next two pictures. This is your o2 sensor.

4. I went to autozone and rented the o2 sensor socket. It's free as long as you return it and it has a notch in the side to allow the wire to pass through, making the job that much easier.

5. I had sprayed the nut with PB blast the day before to loosen it up and I drove the car for 10 minutes prior to starting to warm up the sensor for easier removal.

6. Unscrew sensor, put on new one. Make sure to use anti-seize in case you ever need to replace again.

7. Put everything back together.

Nice job. I have my O2 layin around for my 300sel to install when I can remember to do it.