Testing The Oxygen Sensors -GM 4.3L, 5.0L, 5.7L

Testing the oxygen sensor (commonly referred to as O2 sensors) on your 4.3L, 5.0L, or 5.7L GM vehicle is not that hard to do.

Since oxygen sensors are not cheap and sometimes the PCM (Powertrain Control Module = Fuel Injection Computer) accuses them of being BAD, when they aren't, testing them is a good idea and this article will help you.

Your 4.3L, 5.0L, or 5.7L GM pick up, van , SUV, or mini-van comes equipped with several oxygen sensors, this article concentrates on testing the two that are before the catalytic converter (O2S11 and O2S21).

  1. Important Suggestions And Tips
  2. Symptoms Of A BAD Oxygen Sensor
  3. Oxygen Sensor Basics
  4. Where Are O2S11 And O2S21?
  5. Oxygen Sensor Test 1
  6. Oxygen Sensor Codes Keep Coming Back
  7. More Test Articles

Important Suggestions And Tips

TIP 1: To take advantage of the testing info in this article, you'll need a scan tool. This scan tool must have Live Data capability. Now, you don't need the GM factory scan tool or a professional technician level scan tool since I've written this article for use with a generic scan tool (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. Therefore, take all necessary safety precautions and think safety all of the time.

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 (HO2S11) Circuit Out Of Range Low Voltage (Bank 1).
    2. P0133 Upstream Heated Oxygen Sensor (HO2S11) Circuit Slow Response (Bank 1).
    3. P0151 Upstream Heated Oxygen Sensor (HO2S21) Circuit Out Of Range Low Voltage (Bank 2).
    4. P0153 Upstream Heated Oxygen Sensor (HO2S21) Circuit Slow Response (Bank 2).
  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.

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, the O2 sensors will let it know and the PCM responds by injecting less. If it doesn't inject enough, they'll make it aware of it and the PCM will inject more.

The O2 sensors accomplish this by producing anywhere from 0.100 Volts to a maximum of 1 Volt of direct current that is sent directly to the PCM. The PCM, after receiving these voltages, does its ‘little song and dance’ and translates the voltage amount (depending on the exact voltage the O2 sensor is producing) as either to much fuel is being injected or not enough.

Here are some more specifics:

  1. 1

    If the PCM injects too much fuel, the oxygen sensor responds 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.

  2. 2

    As soon as the PCM sees the Rich Condition it's causing (according to the oxygen sensor's feedback), the PCM injects less fuel. The oxygen sensor immediately reports the change so that the less fuel the PCM injects into the engine, the smaller the voltage that the O2 sensor's voltage produces. Its voltage can go as low as 0.050 to 0.100 Volts.

    This is considered a Lean Condition.

  1. 3

    This process of injecting more or less fuel and the O2 sensors reporting the change to the PCM goes on the entire time the engine is running (and if all of the components are working correctly).

  2. 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.

  3. 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.

If the pre-Cat O2 sensors are BAD, they will not be able to sense how Rich or Lean the exhaust gas is and consequently the PCM will not be able to fine tune the fuel injection to meet emission (pollution) standards. Not only that, your gas mileage will suffer (see: Symptoms Of A BAD Oxygen Sensor).