The oxygen sensors (also called O2 sensors) are one of the most important elements of modern fuel injection systems. A finely tuned fuel injection system with an oxygen sensor can maintain an air/fuel ratio within a close tolerance of .02 percent. Keeping the engine at the stoichiometric ratio (14.7:1 air/fuel ratio) helps the engine generate the most power with the least amount of emissions.
The oxygen sensors are actually located in the exhaust system of the engine, and they sense the oxygen content of the exhaust gases. There are four sensors on the Volkswagen Touareg: one on each side of both of the catalytic converters. The sensors located just in front of the catalytic converter measures the mixture of the exhaust gas exiting the engine. The sensors located in the catalytic converter are used to measure the performance of the converter by comparing the O2 levels before and after. The amount of oxygen in the exhaust varies according to the air/fuel ratio of the fuel injection system. The oxygen sensor produces a small voltage signal that is interpreted by the electronic control unit (ECU) of the fuel injection system. The ECU makes constant adjustments in fuel delivery according to the signal generated by the oxygen sensor in order to maintain the optimum air/fuel ratio.
There are a few signs that your oxygen sensor may be failing. In general, it is difficult to diagnose problems with the sensor, unless all of the other components in the fuel injection system have been checked and determined to be operating correctly. Some of the symptoms of a failed oxygen sensor system are: irregular idle during warm-up, irregular idle with warm engine, engine will not accelerate and backfires, poor engine performance, fuel consumption is high, and driving performance is weak, CO concentration at idle is too high or too low, check Engine Lamp is illuminated.
In general, if the oxygen sensor is not working, the car will be running very poorly, and will also be outputting a lot of harmful emissions. The cars computer will usually give a warning signal that lights up the Check Engine Lamp if the signal received by the computer is out of its normal range. Sometimes the computer may output an error code stating that the oxygen sensor is reading out of range, when in reality the values registered by the O2 sensor are accurate because there is something else wrong with the fuel injection system. Prior to replacing the oxygen sensors, make sure there are no other codes being recorded that may affect the O2 sensor readings.
The sensors are not cheap so make sure you get the trouble codes read and change out the bad sensor. Sensors tend to wear evenly so if they were all replaced at the same time they usually all start failing around the same time so be prepared. One of the things that can bring about the premature failure of the sensor is an improperly running engine, extra fuel or oil in the exhaust gas can destroy a sensor quickly so make sure to check all the engine codes before replacing the sensor or you may just end up having to do it again. Also the pre-cat and cat sensors are different so make sure you are ordering the correct sensor.
The sensors are specific to banks 1-3 and 4-6 and pre and post cats so make sure you order the right ones. The sensors are color coded and the fittings for the pre and post cats are different so it is hard to get them mixed up but the sensors wire length for banks 1-3 and 4-6 are different so pay attention to those.
Even though the motor in our Touareg vehicle is listed as a "V" six both exhaust manifolds are located on the right side of the engine and the O-2 sensors are attached to the manifolds (red arrow).
While you might be technically able to replace the O-2 sensors with the air filter lid in place you should remove it (red arrow); it will give you a lot more room to work and make the job a lot easier. If you are replacing the sensors on bank two (4-6) you will want to remove the air intake tube as well (yellow arrow). Please see our article on replacing your air filter for these procedures.
All of the sensor connections are in a metal clip (yellow arrow) just carefully wiggle and pull them up until they are out, then use a flathead screwdriver to release the clip and separate the connection (red arrow).
It is impossible to tell if the sensor is bad just by looking at it, you will need to trust the error codes on this one. New sensors should come with a small amount of anti-seize on the threads, if yours does not and you are adding some make sure not to get it anywhere but the threads.
If you are replacing the sensors for bank 4-6 you will need to remove the vacuum line that connects to the manifold (yellow arrow) to get access to the wiring connections (red arrow). Use care when moving the vacuum line as it is plastic and gets very brittle with age.