The upstream Oxygen Sensor on your OBD II equipped Jeep 4.0L SUV can be tested, and in this article, I'll show you how in a step-by-step fashion. This tutorial will help you to test the upstream Oxygen Sensor (commonly known as the O2 Sensor Bank 1 Sensor 1: O2S11 and located before the Catalytic Converter). You'll be able to say, "YES, it has failed", or "NO, the problem is something else". The most common symptom, of a failed O2 Sensor, is the check engine light shining nice and bright and OBD II Codes P0131, P0133 stored in the PCM's memory. What sucks is that the PCM may think they're bad when they're really not, so testing them makes a lot of sense.