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Oxygen Sensor Replacement
 
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Pelican Technical Article:

Oxygen Sensor Replacement

Steve Vernon

Time:

2 hour2 hr

Tab:

$760

Talent:

***

Tools:

22mm wrench, 22mm crow's-foot wrench, jackstands

Applicable Models:

Mercedes-Benz ML320 (1998-03)
Mercedes-Benz W203 (2001-07)

Parts Required:

New O2 sensors

Hot Tip:

Take your time with the front 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 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 Mercedes C320: 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 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.

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.

ALWAYS work on a cool car and exhaust system.

Begin by removing the under trays on the front of the car. There are two trays, the first is the engine tray and it is held in place by six 8mm screws, next is the transmission tray. It is also held in place by six 8mm screws but two of them are shared with the engine tray.

The four connectors for the O2 sensors are located at the bottom of the bell housing. Remove the sensor connectors and the wires from their clips.

Separate the connectors by pulling them apart.

For the post cat or rear sensor use a 22mm wrench and simply remove the rear sensor from the exhaust pipe. Both rear sensors are very accessible.

On the front oxygen sensors, you'll need to get a little creative. The front sensors are mounted in a really tight space and at an odd angle. To loosen it, you'll need patience and to use a special type of 22mm crow's-foot wrench with a slit cut in the side. When you remove the O2 sensor, you will probably find that it is coated with black soot. This is normal for an old, worn out O2 sensor and usually indicates a rich running condition.

Install your new sensor snug-tight, and torque to spec. It's also a smart idea to add some anti-seize compound to the threads of the plug before you install it, but make sure the anti-seize doesn't get into any of the slits on the head of the sensor. Check the sensor first though, as new ones usually come with a dab of anti-seize already on the threads.

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.

Jack the car up and secure it on jackstands at all four factory jacking points (please see our article on jacking up and supporting your Mercedes).
Figure 1

Jack the car up and secure it on jackstands at all four factory jacking points (please see our article on jacking up and supporting your Mercedes). You will need to remove the two under trays. Begin by removing the engine tray. It is held in place by six 8mm screws (red arrows).

Next is the transmission tray.
Figure 2

Next is the transmission tray. It is also held in place by six 8mm screws (red arrows) but two of them are shared with the engine tray.

The four connectors for the O2 sensors are located at the bottom of the bell housing.
Figure 3

The four connectors for the O2 sensors are located at the bottom of the bell housing. The yellow arrows show the rear or post cat connectors and the red arrows show the front or pre-cat connectors

Remove the rear sensor connectors (red arrow) and the wires from their clips (yellow arrow).
Figure 4

Remove the rear sensor connectors (red arrow) and the wires from their clips (yellow arrow).

Remove the forward sensor connectors (red arrow) and the wires from their clips (yellow arrow).
Figure 5

Remove the forward sensor connectors (red arrow) and the wires from their clips (yellow arrow).

Separate the connectors by pulling them apart (red arrow).
Figure 6

Separate the connectors by pulling them apart (red arrow).

For the post cat or rear sensor use a 22mm wrench and simply remove the rear sensor from the exhaust pipe.
Figure 7

For the post cat or rear sensor use a 22mm wrench and simply remove the rear sensor from the exhaust pipe. Both rear sensors are very accessible. I used the 22mm craw's foot socket (red arrow) in this picture to show what it looks like. It is such a tight fit getting it on the forward sensors that I could not get a good picture of it.

On the front oxygen sensors, you'll need to get a little creative.
Figure 8

On the front oxygen sensors, you'll need to get a little creative. The front sensors are mounted in a really tight space and at an odd angle. To loosen them, you'll need patience and to use a special type of 22mm crow's-foot wrench with a slit cut in the side.

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Comments and Suggestions:
Jes Comments: Hello,

I had check engine codes P0130O2 bank 1 sensor 1 and P0456 EVAP small leak detected appear together on my 2005 c230k. The car has about 100k miles but runs like new.

I don't notice any difference in performance from before and after the codes so I don't think my 02 sensor is bad.

Any chance these codes could have been caused by bad spark plugs not burning all the fuel/oxygen?
October 20, 2016
  Followup from the Pelican Staff: No, the small leak is a leak in the evap system. And the o2 code is likely a faulty o2 sensor. - Nick at Pelican Parts  
bmadams Comments: Hello and thanks for all your great write ups.
I have a 2002 C320 with 149,000 miles. Shortly after purchasing last year Engine light on and showing codes P0432 and P0422 bad 1st Cats, both sides. I have changed the Plugs a few months ago, nothing else. The cats do not smell and do not rattle. Is there any chance the O2 Sensors are causing this? Both Cats bad seems strange, I have been driving MB for like 10+ yrs and never replaced cats.
August 9, 2016
  Followup from the Pelican Staff: Unlikely. the o2 sensors have to pass a test before the cat tests run. They would have fault codes if they were faulty. - Nick at Pelican Parts  
Reub Deub Comments: P0171
May 2, 2016
  Followup from the Pelican Staff: Code P0171 - System Too Lean

Most likely a vacuum leak or faulty mass air flow sensor. - Nick at Pelican Parts
 
Samkenobi Comments: Hi there,
Season's greetings!

I have just replaced both O2 sensors for both downstream and upstream. 2 days after, the check engine light came back.

Could you advice any possible problems before I visit the workshop again?

Thanks in advance!
December 25, 2015
  Followup from the Pelican Staff: What is the fault code stored? - Nick at Pelican Parts  

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