This is the fault message associated with code 9 of the check engine light in my car ('95 E420). I reset the light and waited till it came back on. It shows code 9 again so I know it is not just a fluke. I am wondering how to go about fixing this problem. I would think that I should first make sure that it is completely connected, and if the light still comes back on, replace it. From what I understand it is like a $12 part. Where exactly is the sensor located? What does it look like? Some assistance would be greatly appreciated.