Originally Posted by treiberg

Some shots from underneath of the O2 sensors:

First pic shows the post-cat O2 sensor and wiring. It is simple to remove and then route the wiring. It is also very easy to remove sensor from exhaust because of ample room. The pic actually shows the connector all the way back to the sensor if you look carefully. The pre-cat looks slightly harder because of location.

Second pic shows a better shot of the post-cat O2 sensor.

Third pic shows the two connectors for the O2 sensors. Red one is pre-cat (mine was fine), and it slips off of a tab then you can disconnect it to check for oil. Took me a few minutes to figure out how to disconnect it. The post-cat sensor is easier and just pulls out from a holder then pulls apart.

Originally Posted by treiberg

This is the story of my 2005 C230K (M271 engine). I had the common problem that these cars can have after about 50K miles, or after 4-5 years of service, where the cam sensors leak oil into the main wiring harness of the car. This thread will detail my experience which led me to a repair that saved thousands of dollars versus the repair my local Mercedes dealership proposed. My car has about 70K miles on it.

Original problem noted was check engine light, with codes P0136 and P0141. Both of these codes point towards problems with the post-cat O2 sensor. Gas mileage was also way down, about 25% less MPG than normal. After a little research on the threads (see articles http://www.benzworld.org/forums/w203...onnectors.html and https://mbworld.org/forums/c-class-w...m-sensors.html and https://mbworld.org/forums/c-class-w...placement.html), I was pretty sure that the problem was oil leaking into my harness from one or both cam sensors. A visual inspection revealed oil dripping from the connector at the cam sensors (just unplug the wiring at the cam sensor near front, top of engine). If you do not see oil drip out, just run finger up inside connector to see if oil is present (see pic 1 below). I also crawled under the car and did a visual inspection of the post-cat O2 sensor (the one downstream of the cat converter) by pulling the connector near the sensor and it a good amount of oil (see pic 2 below) - when this oil makes it to the sensor and trips the codes, you will have to replace it (but it is not very expensive). My pre-cat O2 sensor connector was completely clean, so no oil there yet.

Fortunately for 2003 and 2004 C230K owners, Mercedes has issued a recall campaign, and will completely fix your problem at no cost to you. Unfortunately, for some strange reason, the 2005 model (mine), even with the same part numbers and the same design problem, there is no recall (at least as of December 2010). If you spend any money fixing this, hang on to all receipts, because Mercedes will reimburse you if a formal campaign is ever issued.

Next step for me was to go purchase the parts to resolve problem:
Isolation wires (2), to prevent future leakage, $55 from Mercedes (271-150-27-33)
Cam Sensors (2), to stop the existing leakage, $76 from Mercedes (271-051-01-77)
O-ring (2), installed with cam sensor, $10 from Mercedes (010-997-23-48)
Rubber plugs (2), installed with cam sensor, $5 from Mercedes (271-051-00-32)
O2 sensor, post-cat, $125 from autohaus.az

Total cost: Approx $275

I bought the parts and got ready too do the job. On a whim, I decided to call my local dealership (Sangera Mercedes of Bakersfield) to see if they would be willing to help me, since there was a recall for the identical part. Service advisor told me there was really nothing they could do for me. Called again and asked to talk to service manager, and he told me to bring the car down and they would cover 100% of the cost. Yes!! Took the car down and they called me back and said that the cost to repair was much higher than what they can fix for free; total bill would be just over $4K, of which they were willing to cover $1,250. They claimed the oil had also leaked into the ECU (also called the ECM), and this part alone is $2K - plus they wanted to replace the full harness, which is another costly repair because of the labor involved. Kind of feel that they enticed me to come down so they could charge me a bunch of money, but that's another story. I felt that maybe the harness and connectors and ECU could be cleaned up without replacing everything, but for the dealership, it was "all or nothing". I could elect to do the full repair at nearly $3K, or I could come down and pick up the car. Absolutely no option to just replace the O2 sensor and cam sensors alone, at least without me paying for it. Their rationale is that the problem will come back if you don't replace all components affected by the leak - in truth, they just don't want to spend time on any lower cost options, like cleaning parts. They just replace parts, easy for them and at a nice profit, but at a huge premium for the poor guy who has to pay the bill!

I went to the dealership and inspected the ECU harness connector, but found only a trace amount of oil there and concluded the oil damage was probably minimal at this point. Because the car was running reasonably well, this also led me to believe that the ECU had not been impacted yet. I had the dealership put things back together, and drove the car home and started my "budget" repair. My theory is that if I can prevent further leakage, and if I can carefully clean the connectors, that I can avoid the huge cost of ECM and harness replacement. I also realize that this repair will involve periodic cleaning of the connectors, maybe once every week or two, until I see no further contamination. Keep in mind that oil does not conduct electricity, so small amounts are not instantly deadly to the electronics.

1) Installed the new cam sensors with new o-rings and rubber plugs. Very simple, just need a 8mm inverted torx socket to remove (three screws on each sensor). Third pic below shows new installed cam sensors

2) Installed the new post-cat O2 sensor. This is very easy and just involved removing sensor with a 22mm wrench, and unplugging it from the connector. This is done from underneath the car. I used car ramps on front wheels to easily access that area. You have to remove the rear panel underneath the engine held in place by 6 8mm screws. Also, I used Radio Shack electronics cleaner spray on the connector to get it as clean as possible. By the way, O2 sensors do not last foreever, so replacing it at 75K-100K miles, is probably a good measure to restore lost performance, so even with no harness oil, it might be worthwhile.

3) Cleaning of ECU connector. I sprayed electronics cleaner into the connector on the ECU and cleaned it as good as I could with some q-tips. But the major cleaning effort here was on the harness side of the connector. I would spray cleaner into the connector (the one with about a hundred little pin sockets), let it sit for a few minutes, and then use a clean cloth to wipe it off, then repeat this process. I probably did this 10 times before I was convinced that the connector was clean (evidenced by the color of the residue that was coming out of the connector after spraying it). If you have any evidence of ECU contamination (because of lots of oil pooled in connector, or other check engine codes being thrown), then I have read that some have actually removed the ECU and taken it apart and cleaned it internally. I did not have to do this. Fourth pic shows the location of the ECU, and the connectors that are on top. My front one was dry, but rear one had trace amounts of oil. There are tabs that you need to slide forward (for front one) and rearward (for rear one) to get the connectors off. Fifth Pic shows the connector on the end of the harness that had quite a bit of oil in it. Multiple sprays and wipe downs with electronics cleaner finally produced clear fluid with no more traces of oil.

4) I also found I had a leak at the transmission connector, but this problem is unrelated, and is well detailed in other threads (another common problem!). I fixed this as long as I was doing work under the car.

5) I will install the isolation wires too, but this was not immediately necessary, since it should take at least a year or two until those new cam sensors will start leaking again.

I put things back together, and cleared the codes with my scanner, then started the car. This was about three weeks ago. I have now driven several hundred miles with no check engine light, and with gas mileage significantly improved, so I conclude that the repair was a success. After about a week, I crawled under car and found very slight amount of oil at O2 sensor. I cleaned it up, and it looks like I may need to do this a few times over the next few months until I see no more oil. I have seen no new contamination at the ECU connector after these several weeks.

Another lesson learned here is that an easy periodic inspection of the cam sensor connectors can prevent most of this damage. I believe it probably took months of time for that oil to completely migrate through the harness. Another approach is to just install the isolation wires on your car, even if no leakage is apparent yet. This will completely prevent all migration and future damage, at a cost of under $60.

I really hope this helps some other folks (at least anyone inclined to DIY repairs) to potentially saves thousands in dealership repair costs. Because of the work involved, and the risk of further contamination without replacing the harness, this repair may not be for everyone. But for me, saving thousands of dollars was well the worth the 4-5 hours I have spent on this. I will try report back in another 6 months and give everyone an update on the status.