In some ways, the W210 -- sixth-generation, 1995 to 2003 -- E-Class was the car that brought Mercedes into its modern era. Throughout its production cycle, the E-Class saw not only a return to style for this once notably bland marque, but the introduction of several new engines, powertrains and standard handling and safety features that made the previous generation look Stone Age by comparison. One of these was the now-ubiquitous OBD-II computer diagnostics system, which utilized a pair of oxygen sensors to monitor both the engine and catalytic converter.
Chock the Merc's rear tires. Start the engine and allow it to idle up to at least 120 degrees Fahrenheit -- hot pipes make the sensors easier to remove. Shut the engine down, lift the car's front end with a floor jack and secure it on jack stands.
Locate the sensor you need to remove. Sensor No. 1, the upstream sensor, is in the merge collector between the two exhaust header banks. It's accessible from the top, but difficult to see. Sensor No. 2, the downstream sensor, is in the driver side of the exhaust pipe after the catalytic converter.
Unplug the bad sensor, and remove the sensor from the tube. Under the car, you can use a 22 mm wrench or an O2 sensor socket and ratchet to remove the sensor. Up top, you'll need to use either an O2 sensor socket or a crowfoot attachment to squeeze in between the header tubes.
Coat the threads of the new sensor with O2 sensor antiseize lubricant, and carefully hand-thread the sensor into the exhaust pipe until it fully seats. Use a torque wrench and the appropriate fitting to tighten the sensor to 40 foot-pounds, or 55 newton-meters if you're using a Metric wrench.
Plug the sensor in, lower the car and start the engine. Test-drive the car. If the check-engine light doesn't turn off after a few minutes, take the car to your local auto parts store and use the store's OBD-II code scanner to clear the existing diagnostic codes.
