How to Test the Oxygen Sensor (GM 3.8L)

If the upstream oxygen sensor has failed, one of the most obvious symptoms besides the check engine light (CEL) being on, is really BAD gas mileage.

In this article, I'll show you how to test the oxygen sensor (commonly known as the O2 sensor) without having to replace if first to see if it's good or BAD. Since O2 sensors are not cheap... testing it before you replace it is a really good idea.

Your 3.8L GM vehicle comes equipped with 2 oxygen sensors, this article concentrates on testing the one that's before the catalytic converter (O2S11).

Here, at a glance, are the main points of this article:

  1. Important Suggestions and Tips
  2. Symptoms of a BAD Oxygen Sensor
  3. Oxygen Sensor Basics
  4. Start Your Troubleshooting Here.
  5. Basic Upstream Oxygen Sensor Performance Test.
  6. P0131: O2 Sensor Circuit Low Voltage (Bank 1 Sensor 1).
  7. How to Diagnose Trouble Code P0131.
  8. P0132: O2 Sensor Circuit High Voltage (Bank 1 Sensor 1).
  9. How to Diagnose Trouble Code P0132.
  10. P0133: O2 Sensor Circuit Slow Response (Bank 1 Sensor 1).
  11. How to Diagnose Trouble Code P0133.
  12. Oxygen Sensor Codes Keep Coming Back.
  13. More Test Articles.

Important Suggestions and Tips

TIP 1: A scan tool with Live Data capability is a must to be able to use the info in this article. A simple code reader won't help.

Now, you don't need the GM factory scan tool or an expensive professional technician level scan tool. A generic scan tool will do great and I've written this article with this tool in mind (Don't have a scan tool? Need a scan tool? check out my recommendation: Actron CP9580 Scan Tool Review).

TIP 2: You won't have to remove the oxygen sensor to test it, since the test procedure I'm gonna' show you is an On Car Test of the O2 sensor.

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 (DTCs) lighting up the CEL usually are:
    1. P0131 Upstream Heated Oxygen Sensor (HO2S11) Circuit Out Of Range Low Voltage (Bank 1).
    2. P0132 Upstream Heated Oxygen Sensor (HO2S11) Circuit High Voltage (Bank 1, Sensor 1).
    3. P0133 Upstream Heated Oxygen Sensor (HO2S11) Circuit Slow Response (Bank 1).
    4. P0171 System Too Lean (Bank 1).
    5. P0172 System Too Rich (Bank 1).
  3. Really BAD gas mileage.
  4. Won't pass state mandated emission testing.

Quite a few things can cause the above trouble codes to pop up and light up the check engine light (CEL)... and since testing the O2 sensor is pretty easy, this tutorial will show you how.

Oxygen Sensor Basics

The O2 sensor that this article will show you how to test is the upstream oxygen sensor. This bad boy is located before the catalytic converter and is known as O2S11 which means: Oxygen Sensor Bank 1 Sensor 1.

What does the O2 sensor do? Well, in a nutshell, the job of the upstream oxygen sensor is to report back to the PCM (Powertrain Control Module = fuel injection Computer) whether it's injecting too much fuel or not enough. Too much fuel means that the air fuel mixture is Rich and if not enough fuel is being injected, this means it's Lean.

Here are some more specifics:

  1. 1

    If the PCM is injecting too much fuel into the engine, the oxygen sensor reports back just exactly how Rich the air/fuel mixture is by producing a voltage above 0.500 Volts. This voltage can go as high as 0.900 to 1.0 Volt.

    When the PCM gets this report, it immediately starts to cut back on the amount of fuel it's injecting into the engine.

  2. 2

    As the PCM starts to inject less fuel.. it may go too far and not inject enough. Since the O2 sensor is always watching, it reports this Lean condition to the PCM. Here again the O2 sensor saves the day by reporting a voltage that can go as low as 0.050 to 0.100 Volts.

    The voltages that the O2 sensor produces, in a Lean condition, range between 0.050 to 0.500 Volts (any voltage under 0.500 Volts is considered a Lean condition). When the PCM sees this Lean condition, it starts to inject more fuel.

  3. 3

    This process of injecting more or less fuel, by the PCM, goes on the entire time the engine is running (and if the O2 sensor is working correctly).

  4. 4

    What makes troubleshooting the oxygen sensor so easy, is that you can see it reporting these Lean and Rich conditions in Live Data mode on your scan tool (and this is how I'm gonna' show you how to test them).

  5. 5

    The O2 sensor (as long as it's functioning correctly) will produce a voltage that will switch between a Lean and Rich condition several times every few seconds. So, if the O2 sensor has failed, it won't report any of these changes at all or it will respond very slowly.
