1999 ML320 O2 Sensor - Mercedes-Benz Forum

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post #1 of 7 (permalink) Old 08-28-2010, 07:57 AM Thread Starter
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1999 ML320 O2 Sensor

My girlfriend and I share a 99 ML320 that has a number of cosmetic defects but one big problem, it can't pass the state emissions test. We just moved to Arizona in May and the check engine light was on when we moved so we knew we were going to have problems. A mechanic where we used to live ran a diagnostic and told us that both 02 sensors after the catalytic conveters were bad and it would be 400 bucks plus two and a half hours of labor to fix.

When we got out to Arizona we got ourselves a 90 day extension after failing the emissions test and have since been to another mechanic who has run diagnostics several times. The first time he told us the rear 02 sensors were bad and it would be 600 dollars. The second time he told us it would be 1100 dollars because now a third 02 sensor was bad and to reduce labor cost he may as well replace the 4th one. The third time we went to him was after I took a look under the car and found that the rear driver side 02 sensor was unplugged. We took it back for a diagnosis and were told only the rear two 02 sensors were bad.

I've since talked to several mechanic friends and they've all said the hardest part is getting under the car and getting the 02 sensors off but if I go to a mechanic I'll surely be wasting hundreds of dollars (which I can't afford anyway).

After doing some looking around online I have two real big issues standing between me and doing this myself:

1) How do I know only the rear two 02 sensors are bad if the front 02 sensor reported as good after i plugged in the rear 02 sensor?

2) How do I know ANY 02 sensor is bad if only the rear are throwing up codes? Can the diagnostic distinguish between bad 02 sensors and an actual 02 problem in the exhaust?

Further details: The mechanic here in Arizona has used a laptop program with multiple Mercedes logos (though that proves little) to run both "quick" and "full" diagnostic tests. Each time the only faults have been bad brake pads (replaced) and the 02 sensors.

ALSO: How do I find the chasis # / model code? (have seen some things about x 02 sensor only works with x + chasis or - x chasis and except model code 808 / 809 respectively)

Any assistance would be great! Thank you!
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post #2 of 7 (permalink) Old 08-28-2010, 11:48 AM
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Honestly, the parts are like $180 for the (2) Bosch that are plug n play, slightly less for the universal type that get spliced in:

Your Parts Search Returned 0 Part(s) System&gid=5132@Oxygen Sensor

Any general shop can do that for you on a lift, just takes a lift and a little and the right tool - like 20 real minutes - not big labor dollars at all unless I am missing something here.

Keep the beat !
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post #3 of 7 (permalink) Old 08-28-2010, 02:46 PM
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And to answer the other question the scan tools can differentiate all 4 O2 sensors.
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post #4 of 7 (permalink) Old 08-29-2010, 12:28 PM
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Yeah, autohausaz.com for the O2 sensors. Just go to autozone and have them read the codes, search those codes on this forum. It's probably a simple fix, and assuming the truck has a decent amount of miles on it with original o2 sensors, then that's probably the only problem. You can never rule out that something upstream is causing the failure though, so keep that in mind.
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post #5 of 7 (permalink) Old 08-30-2010, 11:20 AM
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I had the almost the same exact problem w/ my 99'. Failed the Illinois smog test, read the codes with my innova scanner and retrieved four post-cat O2 related codes (I forget exactly what they are, they're posted all over benzworld). I did a little research and purchased the sensors from autohausaz. I found step by step instruction here as well. Although I had a little difficulty removing the old sensors I got it after a little persuasion. Keep in mind, the car wasn't lifted at all. After replacing the sensors I drove for about 2 days to let all of the monitors reset, took to the testing site and PASSED. I've been up and running flawlessly for 4 months without issue. I'm almost at 200K
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post #6 of 7 (permalink) Old 08-31-2010, 01:23 AM
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Also on these rigs the cats are prone to failure which can throw a rear O2 sensor code. I have replaced my drivers side and now have to do the ppassenger side as the "brick" has broke loose and is rattling in the cat.
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post #7 of 7 (permalink) Old 08-31-2010, 05:15 PM
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I would do the following. Get the specific codes. Advance Auto or Autozone can read them for you. The codes will tell you which O2 sensor(s) to change. Order the new sensor(s) from autohausaz. Once you get the sensors. Get under your M class. Make sure that you do this when it is completely cold. Exhaust systems get HOT. Locate your O2 sensors. There will be one before and one after each of the catalytic converters-that makes a total of 4 but you will only be changing the ones that threw a code. Unplug the electrical connector for the sensor. If I remember correctly there is a bracket that holds part of the sensor wiring or something in place. Unbolt the bracket. To undo the O2 sensor itself there is a special tool. I had no need for one. The right size wrench worked fine. Reinstall the new sensor and reattach the bracket. It should not take long.

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