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Oxygen Sensor Replacement
 
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Pelican Technical Article:

Oxygen Sensor Replacement

Steve Vernon

Time:

2 hours2 hrs

Tab:

$95 to $800

Talent:

***

Tools:

22mm wrench, 22mm crow's-foot wrench, jack stands, floor jack, wheel chocks, safety glasses, torque wrench

Applicable Models:

Mercedes-Benz C350 (2008-14)
Mercedes-Benz GLK350 (2010-14)

Parts Required:

New 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 system. 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 four on the Mercedes C350: 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 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.

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.

Begin by safely raising and supporting your Mercedes. Please see our article on safely jacking up and supporting your W204.

With the vehicle safely lifted and supported remove the rear engine/transmission cover by removing the six 8mm bolts (red and yellow arrows).
Figure 1

With the vehicle safely lifted and supported remove the rear engine/transmission cover by removing the eight 8mm bolts (red and yellow arrows). The forward bolts also help hold the middle engine tray in place (yellow arrows).

There are four O-2 sensors on the C350: two pre-catalytic convertors (yellow arrows) and two at the rear of the cats (red arrows).
Figure 2

There are four O-2 sensors on the C350: two pre-catalytic convertors (yellow arrows) and two at the rear of the cats (red arrows). The wiring connections are located on the lower sides of the bell housing (green arrows).

This photo shows the two sensors on the left side, the pre-cat (red arrow) and the cat sensor (yellow arrow).
Figure 3

This photo shows the two sensors on the left side, the pre-cat (red arrow) and the cat sensor (yellow arrow). These sensors are NOT interchangeable. The right side is the same as the left though.

Both sensors connect to the wiring harness on the lower side of the bell housing.
Figure 4

Both sensors connect to the wiring harness on the lower side of the bell housing. The connection for the pre-cat sensor (red arrow) is different from the cat sensor (yellow arrow) so you can't mix them up.

Simply pull the connection for the sensor you are changing out from the retaining clip and separate the connection.
Figure 5

Simply pull the connection for the sensor you are changing out from the retaining clip and separate the connection.

While the size of the base of the sensors is 22mm you really should get and use a 22mm Crawfoot socket.
Figure 6

While the size of the base of the sensors is 22mm you really should get and use a 22mm Crawfoot socket. The sensors are difficult to reach and can get quite stuck. You do NOT want to round off the sensor trying to remove it. I highly recommend using the Crawfoot socket.

The sensors can be difficult to get to so have a series of extensions and universals ready.
Figure 7

The sensors can be difficult to get to so have a series of extensions and universals ready. Use the Crawfoot socket and remove the old sensor.

There is absolutely no way to tell if a sensor has gone bad by looking at it, so make sure you have checked the fault codes.
Figure 8

There is absolutely no way to tell if a sensor has gone bad by looking at it, so make sure you have checked the fault codes. The sensors have different length cables with different connections so while they will screw into both ports. You really, really have to try hard install and connect them incorrectly. The new sensor will have a small amount of anti-seize on the threads. Make sure you do not get this on the sensor itself when installing. Installation is the reverse of removal.

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Comments and Suggestions:
O2mie Comments: So as a follow up to Shade-Tree's question, if the Pre-Cat O2 sensors are showing a flat zero reading at idle, and/or at 2500 RPM, does that mean that those two O2 sensors are bad?

I should also mention that while the above is occuring, the voltage output signal from the post-cat O2 sensors does swing back and forth?

Lastly, would this explain why the check engine light is coming on and is storing codes thay are P0420 & P0430 which are described as "Catalyst System Efficiency below threshold - Bank 1 & Bank 2" respectively?

Thanks in advance...
September 14, 2016
  Followup from the Pelican Staff: The o2s may stop switching at idle due to heat or mixture control. This needs to be checked using a lab scope (viewing the waveform) during a steady cruise.

if you cat codes, the cats are likely faulty. - Nick at Pelican Parts
 
Shade-Tree-Doofus Comments: Any idea on what voltage reading the PreCat O2 sensors are supposed to be reading under normal operating conditions when the vehicle is running exactly as it should be? Thanks for any info.
August 31, 2016
  Followup from the Pelican Staff: Yes, switching from 0 to 1 volt 3-5 times per second. - Nick at Pelican Parts  
O2mie Comments: In the first step, the image showing 6 bolts to remove bottom engine/transmission cover... There are actually EIGHT bolts. Additional 2 indicated by the orange arrows: http://imgur.com/a/1n2Br
August 27, 2016
  Followup from the Pelican Staff: Thanks for the additional info. We appreciate it. I will have the article updated.
- Nick at Pelican Parts
 

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