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Pelican Technical Article:

Oxygen Sensor Replacement

Steve Vernon

Time:

2 hours2 hrs

Tab:

$175 to $375

Talent:

**

Tools:

22mm or 22mm crows-foot wrench, jackstands

Applicable Models:

VW GTI (2006-09)

Parts Required:

New O2 sensors

Hot Tip:

Do not get anti seize on the sensor

Performance Gain:

Car runs better.

Complementary Modification:

Oil Change

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 located in the exhaust system of the engine, and they sense the oxygen content of the exhaust gases. There are two on the GTI MkV: one on each side of the catalytic converter. The sensor located just in front of the catalytic converter measures the mixture of the exhaust gas exiting the engine. The sensor located after the catalytic converter is 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, 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 car's 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.

Always purchase the correct sensor for the car. The wire resistance and proper connection to the harness are vital for the performance of the sensor. Snipping and soldering wires together can affect the resistance in the wire and cause the sensor to give false readings. While the factory sensors are expensive, in the long run you will probably not end up saving yourself any money and end up doing the same job twice if you install cheap generic sensors.

Troubleshooting the complete fuel injection system is beyond this project's scope. If you think that the oxygen sensors may be causing some of your fuel injection problems, they should be replaced. In general, I recommend that you do this as needed. You will have to jack up the car and secure it safely on jackstands to gain access to the sensors. Please see our article on safely jacking up and supporting your vehicle.

Begin by safely jacking up and supporting your car. Please see our article on safely raising and supporting your vehicle for further assistance. ALWAYS work on a cool car and exhaust system.

The heated pre-cat sensor is located right next to the turbo.
Figure 1

The heated pre-cat sensor is located right next to the turbo. The connection for the wiring is located on the firewall just above the brake booster (red arrow). You may be able to access it with your engine cover on but I recommend you remove the engine cover for much easier access. Please see our article on engine cover removal for additional assistance.

Slip the connector (red arrow) from the mount on the firewall (yellow arrow).
Figure 2

Slip the connector (red arrow) from the mount on the firewall (yellow arrow).

Separate the connection.
Figure 3

Separate the connection. Each O2 sensor is specific to its location (pre-cat and post-cat). The wiring connections are different so you can not install the sensor in the wrong place (red arrow).

You are going to need to unclip the wiring from the clips on the firewall (red arrows) but you may want to wait until you have the sensor out of the cat.
Figure 4

You are going to need to unclip the wiring from the clips on the firewall (red arrows) but you may want to wait until you have the sensor out of the cat. It is easier to pull the sensor out from above and lower the sensor into place from above than try to route the wiring up from below.

Working from below the car use a 22mm wrench or crows foot and remove the sensor from the cat (red arrow).
Figure 5

Working from below the car use a 22mm wrench or crows foot and remove the sensor from the cat (red arrow). Installation is the reverse of removal. Lower the sensor down from above, use care not to get anti seize on the sensors end. Thread the sensor in by hand first and then tighten with the wrench.

The post cat sensor is located towards the middle of the car just behind the cat (red arrow).
Figure 6

The post cat sensor is located towards the middle of the car just behind the cat (red arrow). The connection is in a plastic housing along the right side frame rail (yellow arrow).

Remove the connection from the clip (red arrow) and separate (yellow arrow).
Figure 7

Remove the connection from the clip (red arrow) and separate (yellow arrow). Each O2 sensor is specific to its location (pre-cat and post-cat). The wiring connections are different so you can not install the sensor in the wrong place (red arrow).

Working from below the car use a 22mm wrench or crows foot and remove the sensor from the cat (red arrow).
Figure 8

Working from below the car use a 22mm wrench or crows foot and remove the sensor from the cat (red arrow). Installation is the reverse of removal. Use care not to get anti seize on the sensors end. Thread the sensor in by hand first and then tighten with the wrench.

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