2004 Mercedes Benz C230 O2 sensor

First, it is only necessary to change the faulty O2 sensor not both, unless of course they are both failed. First run a engine code scanner on you vehicle to pull the check engine light codes (available at most parts stores). The codes will read something like P0131 Bank 1 O2 Sensor 1 low circuit voltage. Sensor 1 is pre-catalyst, sensor 2 is post-catalyst. The C230 has only two O2 sensors and 1 bank. One before the catalyst and one dirctely after. They are relatively easy to replace from underneath the car. On the right side of the motor (passanger side) you can see the exhaust manifold connect to the exhaust pipe. On the exhaust pipe you should be able to see the first O2 sensor right before the catalyst. The electrical connector for this sensor is connected to the firewall right behind the motor. The connector is a cylinder shaped connector and about the size of a AA battery. The second sensor is right behind the catalyst and can easily be seen from the undercarriage. Follow the wire to the connector mounted on the transmission and disconnect. The O2 sensors require a 22mm wrench to remove and both should be removed from the undercarriage. Also, if you have a rust penetrator, spray the O2 sensors with the exhaust cold and let sit for a few minutes before removing the sensors. This will make removal much easier. Plug in the connectors and clear all codes.