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Oxygen Sensor Replacement
 
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Pelican Technical Article:

Oxygen Sensor Replacement

Steve Vernon

Time:

2 hours2 hrs

Tab:

$160 to $440

Talent:

**

Tools:

22mm wrench, 22mm Crows foot wrench, 10mm socket

Applicable Models:

R53 (2002-06)

Parts Required:

O2 sensors

Hot Tip:

Install the new sensors by hand first

Performance Gain:

Car runs better.

Complementary Modification:

Oil Change

The oxygen sensors (also called O2 sensors) are one of the most important elements of the 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 R53 Mini: one each before and after the catalytic converter. The sensors located just in front of the catalytic converter measures the mixture of the exhaust gas exiting the engine. The sensors located after 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 both replaced at the same time they usually both 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.

After you disconnect the pre-cat sensors wiring harness you will be removing the sensors themselves from below the car so please see our article on safely raising and lifting your R53.

Looking from below on the left side of the vehicle you can see the pre-cat oxygen sensor (red arrow) and the post-cat sensor (yellow arrow).
Figure 1

Looking from below on the left side of the vehicle you can see the pre-cat oxygen sensor (red arrow) and the post-cat sensor (yellow arrow).

If you are going to be replacing the sensors make sure to buy a 22mm Crawfoot wrench, the socket fits easily over the sensor and surrounds it saving you from stripping them out.
Figure 2

If you are going to be replacing the sensors make sure to buy a 22mm Crawfoot wrench, the socket fits easily over the sensor and surrounds it saving you from stripping them out. The wrench is cheap and makes easy work of getting into the difficult places O-2 sensors are usually located.

To remove the pre-cat sensor open the hood and locate the wiring connection on the left side of the motor (red arrow).
Figure 3

To remove the pre-cat sensor open the hood and locate the wiring connection on the left side of the motor (red arrow). Squeeze the tab and disconnect the connection and lower it down so it clears everything else.

Use the 22mm Crawfoot and reach up in between the sway bar and body and remove the sensor (red arrow).
Figure 4

Use the 22mm Crawfoot and reach up in between the sway bar and body and remove the sensor (red arrow).

To remove the post-cat sensor begin by disconnecting the wiring connection (red arrow) and then use a 10mm socket to remove the heat shield nut (blue arrow) and gently pull the heat shield back until you free the wiring all the way back to the sensor (yellow arrow).
Figure 5

To remove the post-cat sensor begin by disconnecting the wiring connection (red arrow) and then use a 10mm socket to remove the heat shield nut (blue arrow) and gently pull the heat shield back until you free the wiring all the way back to the sensor (yellow arrow).

Use your 22mm Crawfoot and remove the sensor.
Figure 6

Use your 22mm Crawfoot and remove the sensor. New sensors will have a small amount of anti-seize on the threads, make sure you do not get it on the sensor itself. Installation is the reverse of removal.

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