How to Change the Downstream O2 Sensor on a 2003 Mercedes W203 Car

You should take your car to an auto-parts store to have it checked with a diagnostic tool when the engine light comes on and the car starts idling erratically. If the diagnostic tool shows a reading of P0141, this means that the downstream O2 sensor on the exhaust system has to be replaced in order to restore the functionality of your Mercedes. This is a part you can replace by yourself if you have some mechanical abilities and you know how to set the car up on jack stands. If your car is running rough and the diagnostic tool shows a reading of P0141, here is how you can change the downstream O2 sensor on a Mercedes W203.

Jack Car Up

The O2 sensor you will be replacing is on the passenger side of the car. The sensor will be easier to reach if you jack up the front of the car and set it on jack stands. Remember to put a block of wood or a brick behind the back wheels so the car doesn't start to roll and fall off of the jack stands.

Remove O2 Sensor

The O2 sensor is located on the front exhaust pipe just past the manifold. Look for the exhaust system and follow it to the front of the engine to find the sensor. Disconnect the wire harness going to the sensor by pulling up on the clip and sliding the harness of the connection on the engine.

The sensor might be rusted and hard to unscrew. Soak the threaded end of the sensor with penetrating oil to eat away the rust. The downstream sensor can be removed with a crescent wrench or with a 22mm socket and ratchet. Take the crescent wrench and turn the sensor counterclockwise to remove it from the front pipe.

Replacement Sensor

The exact downstream O2 sensor-replacement part is called a bank 1 sensor 2 O2 sensor. The original part is made by Bosch, but you can use aftermarket parts if you so choose.

Install New Sensor

Make sure you put the gasket on over the threaded end before you put the sensor into the opening on the tailpipe. Turn the sensor clockwise with your fingers until it tightens. You should use a wrench to finish tightening the sensor.

Test Sensor

Take the car off of the jack stands and go for a long drive. The rough idling should stop right away, and the engine light will go off after the computer cycles—about 20–30 minutes after you start your drive. Once the engine light goes out, you will know everything is now in good working order again. 

Talk to a company like Sterling Service Inc for professional help.