Replacing an oxygen sensor in a Mercedes E320 is not as difficult as determining whether the oxygen sensor is giving the wrong signal because of a problem in the engine management system or if it's the sensor itself. The oxygen sensor is one of the only sensors that can indicate a problem without a failure in the sensor. Most all of the other sensors on the vehicle either work or they don't, and a "Check Engine" light with a corresponding code for that sensor usually means a failure.
An oxygen sensor measures the amount of oxygen present in the exhaust system in real time. Too much oxygen in the exhaust gives a signal for a mixture that is too lean, and the reverse for too little oxygen. Any time that the sensor indicates one of these conditions, the "Check Engine" light will come on and produce a code (for example--oxygen sensor bank 1 sensor 2: O2-B1-S2). It will say "Out of Range." Before a definitive answer can be given as to the functioning of the sensor, the engine management system should be checked first. This will include the ignition system, from crank sensor to spark plug and everything in between--the throttle body for coking, PCV valve, air cleaner, fuel injectors, timing, coolant temperature, fuel pressure, valve timing, vacuum leaks and the emission system as well. If all of this is up to par, it's a safe bet the sensor is faulty.
There are some basic tools needed to complete this job. You will also need a special 7/8-inch oxygen sensor socket with a relief for the electrical harness, and an OBD code reader. Both items can be bought at an auto parts store.
Connect the code reader to the OBD II port under the driver's side dash and turn the key to the "On" position with the engine off. Push the button for READ, and the code reader will interrogate the computer and render a code. Cross-reference the code with the interpretation of the code on the sheet that came with the tool. It will say which sensor is bad. Most Mercedes have four sensors, so the correct one is essential.
Turn off the key, raise the car and support it with jack stands. Disconnect the proper oxygen sensor wiring harness connector. Use either the wrench or the socket, whichever one fits best, and remove the sensor. Before installing the new sensor, put some anti-seize on the threads so it will come out in the future. Install the oxygen sensor and connect the wiring harness. Lower the car and turn the key once more to the "On" position. This time, push the "Clear Codes" button on the code reader and it will reset itself and turn off the engine light.
