The O2 Sensor and You: Why You Need Them

Modern vehicles are much more complex than their predecessors.  Numerous additions have been made to increase fuel economy, as well as to decrease emissions.  One of these things is the O2 sensor.  All modern cars have at least one sensor, though most have two and several have four sensors.  These sensors are located at various points in the exhaust system and play a vital role in both fuel economy and emission regulation.

For the tuner, O2 sensors can be huge pain in the behind. Custom exhausts are a great way to add power and performance to your vehicle.  Unfortunately, you can find yourself without a place for your oxygen sensors if you don't buy the right type of pipes and headers.  Typically, one sensor will be mounted on the exhaust manifold gasket (which is replaced by the header) and the other will be placed midway down the exhaust pipe, usually just before the catalytic converter.

Together, these two sensor help regulate your fuel/air mixture, as well as EGR valve operation.  Without these sensors, your check engine light lights up, your car runs rough and your fuel mileage can significantly decrease.  In addition, it's pretty hard (read as impossible) to pass an emissions test without O2 sensors installed.  That pesky check engine light will usually give you away.

What do oxygen sensors do?  They tell your car's computer the volume of unspent gas fumes in your exhaust gasses.  Depending on this reading, your EGR (Exhaust Gas Recirculation) valve will open or close, allowing those fumes to feed your engine and increasing fuel mileage.  While tuners are not necessarily interested in getting the best gas mileage or reducing their emissions, you still have to have these babies installed to pass your emissions test.  Emissions testing has become mandatory in most urban areas and even in many suburban and rural areas.

In addition to making it impossible to pass the emissions test, the fact that the check engine light remains on makes it difficult to know when another problem has occurred.  The check engine light is designed to illuminate under a plethora of different conditions, from sensor malfunction to transmission problems and even loose gas caps (fuel leak sensor).  The light will not turn off until you have reinstalled those O2 sensors.  So, what are you to do?

The best way to circumvent the problem is to purchase headers and custom exhaust pipes with oxygen sensor bungs already in place.  This way, all you have to do is unscrew the sensors from the original pipes and install them in the new ones.  The check engine light stays off and you stay legal for testing purposes. Oxygen sensor removal will require a special socket, designed to fit the diameter of the nut made into the sensor, as well as unhooking the wiring harness leading into the vehicle.  The harness is usually found a short distance from the sensor; just follow the wire and you will find it.  In some vehicles, the wire actually terminates under the seats, inside the car, so be prepared.