The oxygen sensor actively measures the exhaust output from the engine in order to determine the proper mixture for the car. If you are having trouble adjusting the mixture on your car to proper levels, there is a high chance that you may have a faulty oxygen sensor. The oxygen sensor is located before the catalytic converter because the air/fuel mixture ratio cannot be accurately calculated after the exhaust gases have been processed through the CAT. It's also highly recommended that you check out our Pelican Technical Article on Replacing Oxygen Sensors.