Oxygen Sensor Basics

If you're like me, you want to get right into testing the O2 sensor and forget about having to read the working theory of the component, but in this case it's a good idea to learn how the oxygen sensor works so that you can see the ‘how’ and ‘why’ of the actual test.

The following description (of how the O2 sensor) works is to the point and as brief as possible but without omitting the important stuff you need to know to successfully test upstream oxygen sensor (O2S11).

  1. 1

    The upstream oxygen sensor (O2S11) is tasked with reporting how Rich or Lean the exhaust gas is. This info is sent directly to the PCM. The PCM then uses this info either inject more or less fuel.

    Rich : means that the air/fuel mixture (that's being ignited in the engine) is too fuel heavy. This causes an excess amount of unburned fuel (hydrocarbons) to pass into the exhaust, which cause the O2 sensor to report voltage values above 0.5 Volts.

    Lean : means that the air/fuel mixture is too fuel light. This results in high concentration levels of oxygen to pass into the exhaust gas, which causes the O2 sensor to report Voltages below 0.5 Volts.

  1. 2

    The PCM is constantly adjusting the amount of fuel it's injecting into the engine, which causes the O2 sensor to constantly switch between a Lean and Rich condition.

    So, when the PCM injects too much fuel, the O2 sensor reports voltages above 0.5 Volts (Rich Condition). As soon as the PCM sees this, it injects less fuel.

    As the PCM injects less fuel, it may go too far and not inject enough. The resulting Air/Fuel Mixture (after it gets ignited), causes the O2 sensor to report voltages below 0.5 Volts (Lean Condition). The PCM sees this report and starts to inject more.

    This cycle is repeated constantly several times per so many seconds as the engine runs.

  2. 3

    All of these voltages can be easily observed with a scan tool in Live Data mode and this is how I'm gonna' help you troubleshoot the upstream oxygen sensor on your vehicle.

  3. 4

    When the oxygen sensor (O2S11) fails completely, the oxygen sensor stops reporting the changes in the exhaust and usually stays stuck at some voltage number below 0.5 Volts.

    When the O2 sensor wears out, it does report the changes in the exhaust, but very slowly.

    All of the above malfunctions can be observed with your scan tool.

TEST 1: Inducing A Rich Condition

How To Test The Oxygen Sensor (O2S11) (Mitsubishi 1.8L, 2.4L)

The first thing we'll do, is to see how fast the oxygen sensor responds to a Rich condition (that you will induce yourself).

This can easily be done by spraying a little carburetor cleaner into the engine while it's running.

Since your vehicle is equipped with a mass air flow sensor, you should not disconnect the air duct between it and the Throttle to spray carb cleaner into it (since this will cause the engine to stall).

My recommendation is to spray carb. cleaner into a vacuum hose, that has vacuum when the engine is idling.

Once the carb spray hits the engine cylinders, you'll get an instant Rich condition which will make the O2 sensor respond by producing its maximum voltage (0.900 Volts +) and you'll be able to see this on your scan tool (in Live Data mode).

Alright, this is what you'll need to do:

  1. 1

    Connect your scan tool to your car and get to its Live Data mode.

    Once you're in Live Data mode, scroll down to the PID labeled: 02S11, see the photo in the image viewer above to see what this looks like (if you don't have a scan tool and you need to buy one, check out my recommendation here: Actron CP9580 Scan Tool Review -at troubleshootmyvehicle.com).

    This is the PID that will show you what the O2 sensor is up to as the engine runs.

  2. 2

    Start the engine and let it warm up. Let the engine run for about 10 to 15 minutes.

    The optimal engine temperature range, to get the best result from this test, is to have the coolant at 169° F (76° C). You can verify this coolant temperature with your scan tool also, in the PID that is labeled: Coolant.

  1. 3

    Once the engine has warmed up, take a look at the voltage numbers your scan tool is reading.

    If the engine has been idling for about 15 minutes, the voltage numbers of the O2 sensor should be moving between 0.100 and 0.900 Volts constantly.

    If the voltage value stays fixed, don't worry about this yet and continue to the next step.

STEP 4 Continued in the next page...

Chrysler Vehicles:

  • Sebring 2.4L (SOHC)
    • 2001, 2002, 2003, 2004, 2005

Dodge Vehicles:

  • Stratus 2.4L (SOHC)
    • 2001, 2002, 2003, 2004, 2005

Mitsubishi Vehicles:

  • Eclipse 2.4L (SOHC)
    • 2001, 2002, 2003, 2004, 2005

Mitsubishi Vehicles:

  • Galant 2.4L (SOHC)
    • 1999, 2000, 2001, 2002, 2003, 2004
  • Lancer 2.4L (SOHC)
    • 2002, 2003, 2004, 2005, 2006
  • Mirage 1.8L
    • 1997, 1998, 1999, 2000, 2001, 2002
  • Outlander 2.4L (SOHC)
    • 2003, 2004, 2005