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Discussion Starter · #1 ·
I did something stupid (not the first or the last time, I'm sure) and used dielectric grease on my upstream oxygen sensor connector. I am now well aware that this is a giant NO-NO but I'm trying to recover from my mistake and I'm hoping someone here will have advice and/or insight into the problem. After I first put the dielectric grease (silicone based) on the connector the car ran fine for a few miles but then the CEL popped up with a P2195 code - oxygen sensor stuck lean. The car ran OK but a bit rich since I believe that is the default response when it doesn't trust the O2 sensor reading.

Once I got home I started reading about the dangers of using dielectric grease on O2 sensors since they take their reference air from the connector and the grease will interfere with that process. First thing I tried was to clean both sides of the connector thoroughly with electronic cleaner. I soaked the sensor side connector in liquid cleaner and sprayed the harness side connector many times with the cleaner. I let them dry completely and then reassembled and tested - no difference. I'm not sure how the reference air travels from the connector to the sensor but one thing I read was that it travels in the space between the insulation and the copper wire. If this is true, I thought, then perhaps I've plugged the air supply at the beginning of the wires because of the grease and perhaps cutting 'vents' in the wire further down might provide an unblocked source of air. So I cut off a small piece of insulation from each of the wires leading to the sensor and then wrapped each one loosely in electrical tape. No change. I had a new sensor on order anyway since mine had over 226k miles on it.

I went to a nearby wrecking yard and grabbed one off a 2010 S40 T5 AWD M66 (sad to see it there!) for $10 to use as a guinea pig so I didn't ruin my new O2 sensor when it arrived. Installed it today and once again cleaned the harness side connector with spray cleaner. Sensor worked for a couple of miles before failing with the same DTC. This experience has now led me to several questions I'm hoping someone can help me with:

  1. Did I ruin two O2 sensors because the silicone in the grease is traveling with the reference air and fouling the reference side of the sensor, or is the grease simply plugging the supply of reference air?
  2. How can I thoroughly clean the dielectric grease out of the harness side connector?
  3. Where does the reference air travel from the connector to the sensor?
  4. Is silicone fouling of the sensor element permanent or will it go away with time?
  5. How can I stop doing stupid things in the future? :)
I've thought of two approaches if I can't clean the harness side connector. The first would be to cut the old connector off and splice in a different one. When I got the sensor from the wrecking yard I also cut off the harness side connector just in case it came in handy at a later time. I don't really want to do this since the connector is part of the wiring harness of the car and I don't want splices in it if I can help it. The second would be to create a short connecting bridge by splicing one of the O2 sensor side connectors to the wrecking-yard harness side connector so I could plug a grease fouled O2 sensor side connector into the grease fouled harness side connector and then have the non-fouled wrecking yard harness side connector to plug into another O2 sensor (wrecking yard supplied or new.)

Any thoughts or advice on how I should proceed? Thanks!
 

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If it were mine, I'd junk the entire O2 sensor and its pigtail, and also cut off the connector on the engine harness and splice in a brand new one. The grease is very likely on both halves of the junction, and down the air channel. Being silicone, there's basically no getting it out.
 

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Discussion Starter · #3 ·
Thank you for your input. Just to wrap this up (for now), I still didn't feel like cutting and splicing the harness side connector so I took one of the wrecking yard harness side connectors and connected it to the existing harness side connector wire-by-wire and wrapped it all in heat shrink tubing. Working beautifully a week later.
 

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I just replaced my O2 sensor and thanks to this post, I knew not to apply dielectric grease to the connector. I wasn't getting any codes but my fuel economy has been poor the past several months and the long term fuel trim is a little high at +7%. The car has 148K miles and I had a new O2 sensor that never got installed in my previous V50. The old sensor was very orange looking, which is often associated with fuel additives. I don't use additives often but I do mainly run Shell premium gas. Long ago I recall carborators with orange deposits from Shell gas. The jury is still out on the fuel economy but seems better. The LTFT is still running +7%. The car runs great, improved since changing the O2 sensor.
 

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Discussion Starter · #5 ·
I'm just happy relating my ordeal helped someone else avoid the same mistake! I'm running with the second wrecking yard O2 sensor I installed with unknown mileage on it and the car is running great. I'm saving the brand new O2 sensor for when I finally install my downpipe.

Funny story - prior to my dielectric grease fiasco I ordered a Bosch 15176 sensor from RockAuto since the car was running rough and the readout from the sensor seamed a bit strange. I paid extra to get it faster since I wanted to install it by the coming weekend but when I checked the box the night before I was going to install it I found they had sent me the wrong part! It was in the right box with Bosch 15176 stamped on it but the actual sensor in the box was a Bosch 15857 based on the numbers stamped on it and visual identification using online images. I knew it was wrong as soon as I saw the large rubber grommet mid-way along the sensor wires. RockAuto sent the correct part but it came a week later, thus inadvertently saving me from ruining a brand new sensor when I put dielectric grease on the connector. The wrecking yard sensors were $10 each compared to $90 or so for the new one. Whew!
 

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Depending on your car, it may have two or three O2 sensors, be aware.

Positive fuel trims indicate a lean condition, which could mean a tiny air leak, or maybe a slightly sticky injector. I'd certainly try a Techron treatment or two.
 

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kjhiggins - great story about the wrong part delay.

I tried Techron a while back with no change. Since the car idles and runs very smooth, I an doubting that it is an injector issue. I suspect an air leak but where? Anyone know if the IC is probe to developing leaks?
 
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