How To Test The Oxygen Sensor (Jeep 4.0L)

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. S testing them makes a lot of sense.

En Español You can find this tutorial in Spanish here: Cómo Verificar el Sensor de Oxígeno Delantero (4.0L Jeep) (at:

Important Suggestions And Tips

TIP 1: To effectively test the O2 sensor, on your Jeep 4.0L SUV, you'll need to use a scan tool with Live Data capability.

You don't need the Jeep factory scan tool or an expensive professional technician level scan tool to follow the test procedures in this article, since a simple generic scan tool will do just fine (Don't have a scan tool? Need a scan tool? Check out my recommendation: Actron CP9580 Scan Tool Review).

TIP 2: No need to remove the O2 sensor from the vehicle to test it, since this guide will teach you how to test it in action and installed in its place on your Jeep.

Symptoms Of A Bad Oxygen Sensor

The effects of a bad oxygen sensor can be very subtle since they usually do not cause serious drive-ability problems. Here are the most common symptoms:

  1. The check engine light (CEL) will be illuminated on your instrument cluster.
  2. Diagnostic trouble codes lighting up the CEL usually are:
    1. P0131 Upstream Heated Oxygen Sensor (O2S11) Circuit Out Of Range Low Voltage (Bank 1).
    2. P0133 Upstream Heated Oxygen Sensor (O2S11) Circuit Slow Response (Bank 1).
  3. Really bad gas mileage.
  4. Won't pass state mandated emission testing (smog check).

Where To Buy The O2 Sensor And Save

The following links will help you to comparison shop the front oxygen sensor:

Not sure if the above O2 sensor fits your particular 4.0L Jeep? Don't worry, once you get to the site, they'll make sure it fits by asking you the specifics of your vehicle. If it doesn't, they'll find you the right one.

Oxygen Sensor Basics

To effectively diagnose the oxygen sensor (and to understand how to test it), you need to know just a little bit about how it works.

In a nutshell (cause we don't need to get too deep into theory): the oxygen sensor is tasked with the job of reporting if the PCM (Powertrain Control Module = Fuel Injection Computer) is injecting too much fuel or not enough. With the O2 sensor's feedback, the PCM in your Jeep, can fine tune the amount of fuel that's being injected into the cylinders.

Why? Well, because this fine-tuning helps to control emissions and improves gas mileage. How? By reporting if the air fuel mixture is either Rich or Lean.

In layman's terms, Rich means that the PCM is injecting too much fuel for the amount of available air entering the cylinder and Lean means that not enough fuel is being injected. It's as simple as that and the way they work is just the same.

Here are some more specifics:

  1. 1

    As the engine in your Jeep runs, its PCM is constantly injecting fuel and constantly adjusting the amount.

    If it injects too much, the air/fuel mixture is considered Rich and when this happens, the oxygen sensor reacts by producing a voltage above 0.500 Volts. Depending on how Rich the air/fuel mixture, this voltage can go as high as 0.900 to 1.0 Volt.

    As soon as the PCM sees this, it starts to inject less fuel.

  2. 2

    As the PCM starts to inject less fuel, it may go too far and not inject enough. This causes the air/fuel mixture to go Lean and makes the O2 sensor produce and report a voltage below 0.500 Volts. Depending on how Lean the air/fuel mixture is, the O2 sensor's voltage can go as low as 0.050 to 0.100 Volts.

    When the PCM sees these voltage numbers, it knows to inject more fuel.

  3. 3

    If the O2 sensor (O2S11) is working correctly, it will constantly switch between a Lean and Rich condition several times every couple of seconds.

    If the O2 sensor has failed, it will stay stuck at either a Rich or Lean condition and the PCM will not be able to adjust the amount of fuel injection.

  4. 4

    Although it definitely sucks that you Jeep has a problem, the awesome thing is that you easily observe the O2 sensor's activity (to determine if it's bad or not) with a scan tool in Live Data mode, and this is how I'm gonna' show you how to test them.

OK, let's turn the page and let's get testing.

Jeep Vehicles:

  • Cherokee 4.0L
    • 1996, 1997, 1998, 1999, 2000, 2001
  • Grand Cherokee 4.0L
    • 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003, 2004
  • Wrangler 4.0L
    • 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003, 2004