DIY W202 Oxygen Sensor Replacement

Whether you like it or not, it is highly encouraged that you change upstream oxygen sensors every 100K miles.
Read all about oxygen sensors here:
http://www.autohausaz.com/html/emissions-oxygen_sensors.html
I'll summarize:

1. The oxygen sensor (previously called the "lambda sensor")  monitors the level of oxygen (O2) in the exhaust so that the ECU can regulate the air/fuel mixture to reduce emissions.
2. As oxygen sensors age, they become sluggish, so they are not quick enough to respond to changes in air/fuel mixture. The voltage they produce is also not as large as they should be, giving the false impression that the fuel is leaner than it actually is, which causes higher gas consumption and higher emissions.
3. An additional consequence of any oxygen sensor failure may be damage to the catalytic converter. A rich operating condition causes the converter to run hotter than normal. If the converter gets hot enough, the catalyst substrate inside may actually melt forming a partial or complete blockage.
4. Second generation heated oxygen sensors used on mid-1990s and newer vehicles are recommended to be changed every 100K miles.
5. Increased fuel consumption, driveability problems (hesitation or surging), "Check Engine Light" lit or emissions test failure could all be signs of an oxygen sensor in need of replacement.

At 120K miles, I've already seen enough roughness and surges at low speeds to suggest incorrect air/fuel mixture. Considering the fact that I've already changed my Mass Air Flow (MAF) sensor, what's left is the front oxygen sensor, which together with the MAF are used by the ECU to do closed-loop control of the air/fuel ratio.
As for the rear (downstream) sensor, it only needs to be changed if it throws a code or the Check Engine Light turns on.
So here's how to replace a front oxygen sensor for my 1999 C230 Kompressor.

1. Buy a new OEM front oxygen sensor (~$100)
2. Buy an oxygen sensor removal socket (I got the larger one).
3. Loosen and remove the oxygen sensor from the exhaust manifold. It may need to be loosened by heating the exhaust manifold and/or using WD-40 or Pb-Blaster. If any of these compounds are used, they should be cleaned off before fitting the new sensor on to avoid contaminating the new sensor. Heating the exhaust manifold can be done by turning on the engine and idling for two minutes at a time (one or more times) until it just hot enough to remove the sensor.
4. Cool down before installing the new sensor.
5. Install the new sensor with sensor-safe anti-seize for easy removal the next time you change it. Some sensors come with enough lubricant from the factory to save you the trouble. Torque the socket to 55 Nm (44 lbf ft).
6. Tie the new sensor (at the plug end) to the old sensor (at the sensor end) and pull on the old sensor (plug end) from the top and swap out the new sensor with the old one at the wiring harness leading to the ECU.
7. Make sure the wire doesn't touch any hot surfaces (like the exhaust manifold).

Additional Info: 

http://www.mbclub.co.uk/forums/engine/41615-w202-c230k-lambda-sensor-when-should-you-replace.html

http://www.benzworld.org/forums/r170-slk-class/1277304-upstream-oxygen-sensor.html

http://www.mbworld.org/forums/c-class-w203/206488-diy-m271-o2-sensor-replacement.html

http://www.benzworld.org/forums/r129-sl-class/1421450-diy-oxygen-sensor.html

Additional Notes: The coolant temperature sensor is also a good sensor to replace at this mileage. It also affects the ECU's decision to control the air/fuel mixture.

Date: 12/30/2010

Mileage: 120K

Cost: $100

Tool Overhead: $12

Time: 20 minutes