Recently bought a 2003 W203 C180 Kompressor Elegance. On my pre-purchase inspection, I noticed the check engine lamp on. I quickly slipped my Bluetooth OBD2 adapter into the port under the dash and fired up TORQUE on my android phone. It came up with code P0141 - O2 Sensor Heater Circuit (Bank 1 Sensor 2). The car also exhibited the classic poor idle quality associated with an O2 sensor failure. I told the seller the vehicle had major issues with the engine management systems and would need to be repaired if I was to consider purchasing the vehicle.

Move forward a few weeks and the seller rang me. Would I be interested in buying the vehicle if she gave me a discount for the issue with the check engine light? I had since Googled the problem and new it would cost between $100-$200 to have it replaced. I offered her $1000 less than the asking price and she agreed. I couldn't get over to pick the vehicle up quickly enough! Funnily enough, she managed to get a dodgy roadworthy certificate (needed for the transfer of ownership in Victoria-Australia) even with the light on

On getting the vehicle home I once again connected my bluetooth OBD2 adapter and fired up TORQUE. I cleared the code (and many others). But the code kept returning on every restart. I ordered a brand new O2 sensor along with the special tool from Ebay for the total of $62 AUD. That's a lot less than $350 the local shop quoted. Here is the repair.

• Make sure engine is cold because the Oxygen (O2) sensor is screwed into the exhaust.
• Jack up car at front using centre jack point. Place jack stands under jack point on sills.
• Remove rear lower engine cover (6 x 8mm screws)
• Unplug rear O2 sensor 4-pole electrical connector (the most rearward one) and remove using correct 22MM O2 sensor socket and a 1/2" breaker bar. (It will be tight - that is why you need the correct tool).
• Fit new O2 sensor (make sure you use new gasket/ring)
• Reconnect electrical connector.
• Refit lower rear engine cover
• Turn ignition to run position (do not start) and cancel codes using TORQUE on my phone and Bluetooth OBD2 adapter.
• Start car - Viola! No check engine light or rough idle.
• Jack up car and remove stands.
• Take for a drive. OK.
• Feel smug about the $938 I saved!

When I removed the lower engine covers I noted how dry and free of oil leaks the whole under engine/transmission area was for a car that has travelled over 170K KM. Not a drop of oil, not even a smear! It is 100% completely dry! Yet the electrical connector to the O2 sensor was full of engine oil. Weird. I can only surmise that some oil had been spilt during an oil change and filled the connector. I cleaned it out using brake cleaner and then isopropyl alcohol. I was suspect this was the reason for the failure but when I put a multimeter across all the terminals of the old sensor one wire had no circuit with any of the others. Turns out the heater circuit was open.

I am a motor mechanic by trade so tackling this kind of thing, armed with Google, does not faze me. I write this up in the hope that it may help a noob or non-technical person save a few dollars. Oxygen sensors wear out and this is a normal failure for a car of this age & kilometerage,

It is a 2 beer job.

 

After perusing these forums I have found out the oil I found in the O2 sensor connector is a well-known issue and can be traced back to leaking camshaft sensor/magnets. Apparently the oil wicks along the wiring harness all the way down to the O2 sensor and may even get to the ECU and cause damage! Will need to investigate further.

 

thank you for posting !