How To Test The Oxygen Sensor (GM 3.1L, 3.4L)

You're gonna' be surprised just how easy it is to diagnose the oxygen sensor (O2 sensor) on your 3.1L, 3.4L OBD II equipped GM Vehicle.

Testing them will save you some bucks ($$$), since so many different conditions can trick the PCM (Powertrain Control Module = Fuel Injection Computer) into thinking the oxygen sensor is fried.

Since O2 sensors are not cheap, testing them before you replace them is a no-brainer, and I'll show you how in this article.

Your 3.1L, 3.4L GM vehicle comes equipped with 2 oxygen sensors, this article concentrates on testing the one that's before the catalytic converter (O2S11).

Important Suggestions And Tips

TIP 1: To take advantage of the testing info in this article, you'll need a scan tool with Live Data capability, so this rules out a Code Reader. 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).

TIP 2: The testing procedure I'm gonna' show you here, is done with the oxygen sensors in action. Which means that you'll be testing them with the engine running. You will not 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:
   • P0131 Upstream Heated Oxygen Sensor (HO2S11) Circuit Out Of Range Low Voltage (Bank 1).
   • P0133 Upstream Heated Oxygen Sensor (HO2S11) Circuit Slow Response (Bank 1).
3. Really bad gas mileage.
4. Won't pass state mandated emission testing.

Oxygen Sensor Basics

In this section, I'm gonna' go into some working theory of how the O2 sensor works in plain English.

If you need a more technical and detailed explanation, you can Google it, since I'm only gonna' concentrate on the basics you need to know to diagnose them.

In a nutshell, the job of the pre-catalytic converter oxygen sensors is to help the PCM fine tune the amount of fuel that's injected into the engine.

If the PCM injects too much (known as a Rich condition), the O2 sensors will let it know and the PCM responds by injecting less. If it doesn't inject enough (known as a Lean condition), they'll make it aware of it and the PCM will inject more.

Here are some more specifics:

1. If the PCM is injecting too much fuel into the engine, the oxygen sensor produces a voltage above 0.500 Volts. This voltage can go as high as 0.900 to 1.0 Volt.
   
   With this type of voltage, the PCM considers that the air/fuel mixture as too Rich and starts to inject less.
2. As the PCM starts to inject less fuel, it may go too far and not inject enough. Here again the O2 sensor saves the day by reporting a voltage that can go as low as 0.050 to 0.100 Volts.
   
   Voltages in this range let the PCM know that the air/fuel mixture is too Lean and starts to inject more.
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. 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.
5. 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.

The cool thing is that testing the O2 sensor's performance is not hard. Let's turn the page and get testing.