How To Test The Upstream Oxygen Sensor (Dodge 3.9L, 5.2L, 5.9L)

With the help of this article, you'll be able to easily test the oxygen sensor on your OBD II equipped Dodge 3.9L, 5.2L, or 5.9L pick up, van or SUV.

Testing them, before replacing them, makes a ton of financial sense since many times the PCM (Powertrain Control Module = Fuel Injection Computer) accuses them as being BAD when they are not. When this happens... you usually have another issue that's confusing the PCM into thinking they're BAD.

Your 3.9L, 5.2L, 5.9L Dodge pick up, van, or SUV comes equipped with more than one oxygen sensor, this article concentrates on testing the oxygen sensor or sensors that are before the catalytic converter (O2S11 -and O2S21 if equipped).

Important Suggestions And Tips

TIP 1: You'll need a scan tool to use the info in this article. You don't need the Dodge factory scan tool or an expensive professional technician level scan tool... just a generic scan tool with Live Data capability. (Don't have a scan tool? Need a scan tool? check out my recommendation: Actron CP9580 scan tool).

TIP 2: The way I'm gonna' show you to test the oxygen sensors, is with them in place (on the exhaust pipe) and in action, so you do not need to remove them to test them.

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. The 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. P0151 Upstream Heated Oxygen Sensor (O2S21) Circuit Out Of Range Low voltage (Bank 2).
    4. P0153 Upstream Heated Oxygen Sensor (O2S21) Circuit Slow Response (Bank 2).
  3. Really BAD gas mileage.
  4. Won't pass state mandated emission testing.

Oxygen Sensor Basics

To better understand what exactly it is that you'll be testing for, I'm gonna' go into a little working theory of how the oxygen sensors work on your 3.9L, 5.2L, 5.9L Dodge pick up, van or SUV.

If you need a more detailed and technical explanation.. you'll need to Google it, since the following is just the info you and I need to test them.

Here are some more specifics:

  1. 1

    The O2 sensor's job is to help the PCM fine tune the fuel that's injected into the engine. They do this by producing a variable voltage of 0.100 to 0.900 Volts that the PCM uses to interpret that the fuel mixture is either too rich or too lean.

  2. 2

    As the engine is running and if the PCM injects too much fuel, the oxygen sensor reacts by producing a voltage above 0.500 Volts. This voltage can go as high as 0.900 to 1.0 Volt.

    This is considered a rich condition.

  3. 3

    When the PCM sees that it's causing a rich condition, it starts to inject less fuel. The oxygen sensor immediately reports the change and produces a voltage can go as low as 0.050 to 0.100 Volts.

    Any voltage (that the O2 sensor produces) that's below 0.500 Volts is considered a lean condition.

    And so when the PCM sees that it's causing a lean condition... it starts to inject more gasoline.

  4. 4

    This process of the O2 sensors reporting lean and rich conditions and the PCM compensating by injecting more or less fuel happens the entire time the engine is running (and if all of the components are working correctly).

  5. 5

    The really cool thing is that you can observe these changes (using a scan tool in its Live Data mode) and this is how I'm gonna' show you how to test them.

  1. 6

    A correctly working O2 sensor will produce a voltage that will switch between a lean and rich condition several times every few seconds. So, if the voltage output of the O2 sensor stays fixed (when testing it)... the O2 sensor has failed.

If the pre-cat O2 sensors (known as the O2S11 and O2S21) have failed, they won't report how rich or lean the exhaust gas is and so the PCM won't be able to fine tune the fuel injection.

Two of the most common consequences of failed oxygen sensors is higher pollution emitted from the tail pipe and BAD gas mileage (see: Symptoms Of A BAD Oxygen Sensor).