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Pelican Technical Article:

Oxygen Sensor Replacement

Steve Vernon

Time:

1 hour1 hr

Tab:

$175 to $375

Talent:

**

Tools:

22mm or 22mm crows-foot wrench, jackstands

Applicable Models:

Mercedes-Benz W124 (1986-95)

Parts Required:

New O2 sensor

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 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 sensor is located in the exhaust system of the engine, and measures the amount of oxygen in the exhaust gases. This amount of oxygen will vary 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 sensor, 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 sensor may be causing some of your fuel injection problems, it 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 jack stands to gain access to the sensors. Please see our article on safely jacking up and supporting your vehicle.

ALWAYS work on a cool exhaust system.

The O-2 sensor runs from the exhaust pipe through the body to the connection below the right side seat.
Figure 1

The O-2 sensor runs from the exhaust pipe through the body to the connection below the right side seat. Begin by removing the carpeting on the front right side and locate the wiring connection (red arrow).

The factory O-2 sensors have a single plug and three pin connections (red arrow).
Figure 2

The factory O-2 sensors have a single plug and three pin connections (red arrow). There has been plenty of discussion about buying a cheaper generic or off brand sensor and cutting the wires and wiring it up to the old sensors connections. While we do not recommend going that route if you choose to these are the connections you will need to cut off the old sensor.

Follow the sensor wiring and remove the grommet from the chassis (red arrow) so you can pass the wiring through.
Figure 3

Follow the sensor wiring and remove the grommet from the chassis (red arrow) so you can pass the wiring through.

Working on the exhaust before the catalytic converter you will find the single O-2 sensor (red arrow) and the wiring that runs to the interior of the car (yellow arrow).
Figure 4

Working on the exhaust before the catalytic converter you will find the single O-2 sensor (red arrow) and the wiring that runs to the interior of the car (yellow arrow).

While the O-2 sensor on the W124 is easily accessible it still doesn't hurt to have a special 22mm crowfoot socket for removing O-2 sensors in your tool box.
Figure 5

While the O-2 sensor on the W124 is easily accessible it still doesn't hurt to have a special 22mm crowfoot socket for removing O-2 sensors in your tool box.

Use either a 22mm wrench or the special Crowfoot socket and remove the sensor from the pipe (red arrow).
Figure 6

Use either a 22mm wrench or the special Crowfoot socket and remove the sensor from the pipe (red arrow).

With the old sensor removed (red arrow) you can pull the wires out through the chassis and then install the new sensor.
Figure 7

With the old sensor removed (red arrow) you can pull the wires out through the chassis and then install the new sensor. Most sensors will come with a small amount of anti-seize already on the threads. Thread the sensor by hand until you are sure it is seating correctly. Then tighten with your tool. Never spray any solvent, anti-seize or Loctite on the sensor itself as it will ruin it. Run the wires through the chassis and seat the grommet in the body opening; connect the wiring and reinstall the carpet.





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Comments and Suggestions:
Ron Comments: Jack up the driver's side for better access to the sensor.
June 17, 2015
  Followup from the Pelican Staff: Thanks for the additional info. We appreciate it.
- Nick at Pelican Parts
 

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