Oil Leak leading to Bad O2 sensor
 

I've 2003 c230 with 97k miles on it. I've replace both O2 sensor last year due to check engine light indicating it was bad O2 sensor. I guess you are suppose to replace it every 100K miles or so. After about a year later I've got a same error message to replace Pre-Catalytic O2 sensor.
When the mechanic lift the car and check the connector underneath the car, there was oil in the connector for the O2 sensor. This happen last year as well.
The mechanic can not figure out where the oil is leaking from.
The connector is in the same position as the oil filter is located except its underneath the engine (on the driver side).
There was no oil on the wiring of the O2 sensor but there was oil within the plastic part of the O2 sensor.
I guess the oil got onto the plastic part of the sensor and leak into the connector.

Anybody had similar problem?
I hate to replace another O2 sensor and find this will happen again within a year.
Any input will be appreciated.
Thanks.

its possible that its the oil, but not likely. clean out the oil and clear the code. it is common to have to replace the O2s. but you shouldnt have to replace the same one again.

Thank you Chassis221 for responding to my question.
We've already cleared the code and I've ordered the part online.
After 2 days of driving, the code came back.
The main concern is that when we replace the new O2 sensor, this will happen again(oil entering the O2 socket and
ruining the O2 sensor).

One of possibility the mechanic has suggesting was that when another mechanic was changing oil for the vehicle, he was not cafeful enough that oil filter canister's oil was dripping down and gather in the O2 sensor socket.

One other clue might be that I have to add 1/2 quart between oil change(every 5000 miles). Which indicates there is oil leak somewhere in the engine.
Plastic cover underneath the engine is very clean (without any oil streak or drips).

Only oil we found was in the O2 socket(4-5 drops of oil came off from it when we
unplugged the O2 sensor).

Any other thoughts or suggestion would be greatly appreciated.
Thank you and have a wonderful day.

Oil from changed oil filter leaking onto o2 sensor
 

I just had my oil changed last week, and the same thing happened to me. The mechanic was not careful enough not to drop oil onto the sensor when changing the oil filter (located above the o2 sensor). About 2 days later the check engine light came on, and I also bought a new sensor to install. When I was removing the connector, it was soaked with oil. I did my best to clean it up, using compressed air to blow the oil out and wiping the area down with a rag (including the oil filter casing). The check engine light went out for 2 more days, and has now reappeared. I am having trouble trying to remove the connector to achieve a more complete cleaning of the connections. Does anyone have pictures of how to remove the connector (my vision is not the best for close up viewing) There seems to be another cover over what seems to be a fuse in that connector ?
Thank you.

The oil you are finding in the O2 sensor is not from an external oil leak but its oil leaking through the engine wiring harness. The oil starts at the cam shaft adjuster and works its way through the harness to the o2 sensor. Seen it quite a few times. It can also leak into other sensors and the engine control unit. All of the connectors and sealed very well from the outside world.

"Oil Leak leading to Bad O2 sensor"
 

I'm pretty sure it was the sloppy oil filter change that caused the connector to get soaked, I never use oil and had no spots in the garage until this oil change. This is a 4 cylinder model, and the arrangement of the oil filter over the sensor wasn't a real bright idea from Mercedes. I'll be under there cleaning it out today, and will try to take some pictures.

Well, I finally was able to get back under the car and removed the bracket that holds the mating points of the connectors together. After I removed the bracket, I was able to see the clip that held the connector on. I removed the mating connectors, and as you said there was a rubber gasket there, but it did not keep the oil out (pictures attached). I cleaned the oil with some Simple green (mild detergent), blew compressed air into it, and then some connector safe CRC cleaner. (and again some compressed air). I have not tried it to see if the light stays out, but will post success or not next week. It definitely had motor oil in the connector, and I believe that is the cause of the o2 sensor failure.

I have a similar issue on a Mercedes-Benz W208 2001 CLK 200 Kompressor, I have a faulty camshaft magnet connector at the front of the engine which leaked oil into the wiring harness up to the ECU, and then back to the O2 sensor wires down to the exhaust where is located. Fried 2 sensors, on the genuine one, the heater, on the new Bosch replacement, the sensor element. STAR diagnosis reports sensor circuit open/shorted to ground and lean mixture. I thinked of a way to revive the oiled ones somehow.. I know they breathe from the space between wire and insulation, so at least as a try, i thinked of cutting the insulation and replacing it with a new one, and maybe in time the oil inside will get lost.

Hey Benz Lovers-

I have heard of this issue in the early year C230's. The oil gets on the wiring and starts shorting things out. They actually had a quiet recall on my 04 C230K with similar issue. Mine was leaking but they fixed it anyway. Not sure if the earlier years were part of the recall.

Later

Update to C230 Oil in o2 sensor connector
 

After changing the o2 sensor (as seen in thread), 2 days later the car starting running rough at idle. And the o2 Code and a couple other codes relating to misfire in cylinder 1 and 4 popped up. Thinking I had bought a defective o2 Sensor from Mercedes I took it to the dealer, turns out there was a recall on the car from Mercedes, they replaced free of charge:
Camshaft adjustment solenoid, and install a adapter harness invoice text below:
CAUSE: 05900107 Replace cam solenoid and install adapter harness
026628 operations: camshaft adjustment solenoid
replace acc. no.: 05 900 10, Code 0903P05A71
6324 WMB
2 271-051-01-77-05 Magnet
2 271-051-00-32-05 Closing Cove
2 271-150-27-33 starter line
2 002-997-24-90 cable tie
2 010-997-23-48 seal ring
Also changed a Vent Hose 1 271-010-00-82-05
Venting what, I really don't know ?
It runs better than ever now, much smoother. I was disappointed that I couldn't fix it myself.. but free is free you know !