Oxygen Sensor Basics

You're probably in a hurry to get testing... but I recommend you read this section first. Here you'll find out how the upstream oxygen sensors work. This info will help you to see the 'why' and 'how' of the test at the bottom of this page.

In a nutshell, the oxygen sensor's purpose in life is to monitor the exhaust to see if the PCM is injecting too much fuel or not enough (into the engine).

If the PCM is injecting too much fuel... this is considered a Rich Condition. If the PCM does not inject enough fuel, this is called a Lean Condition.

By reporting how Rich or Lean the exhaust is, the O2 sensor or sensors help the PCM fine tune the amount of fuel the engine need. All this fine tuning helps to cut down on the amount of pollution the engine creates (and give the best fuel economy).

Let me go into more specifics:

  1. 1

    When the PCM injects 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.

    When the PCM sees this Rich Condition, it starts to inject less.

  2. 2

    As the PCM starts to cut back 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. 3

    If the oxygen sensors are working correctly, they'll switch between a Lean and Rich Condition several times every few seconds the whole time the engine is in operation.

    If the O2 sensor fails... it will stay stuck at a certain voltage output and not respond to the changes in the air/fuel mixture.

  4. 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.

Where Are 02S11 And O2S21 Located?

How To Test The Front Oxygen Sensors (4.0L Ford)

Your specific 4.0L Ford vehicle may be equipped with up to 4 oxygen sensors and locating them can be confusing.

The two upstream oxygen sensors (O2S11 and O2S21), that this article will show you how to test, are located before the catalytic converter.

The key to knowing their location is understanding where Bank 1 and Bank 2 are located.

O2S11 (Oxygen Sensor Bank 1 Sensor 1).

  1. Bank 1 is the engine bank that houses cylinders 1, 2, and 3.
  2. O2S11 will be located on the exhaust pipe that connects to the exhaust manifold for this bank.

O2S21 Oxygen Sensor Bank 2 Sensor 1.

  1. Bank 2 is the one that contains the cylinders 5, 6, and 6.
  2. O2S21 will be located on the exhaust pipe that connects to the exhaust manifold for this bank.

When the O2 sensors are located after the catalytic converter, they are referred to as sensor 2... as in O2S12 and O2S22.

O2 SENSOR TEST 1: Inducing a Rich Condition

How To Test The Front Oxygen Sensors (4.0L Ford)

In a nutshell, what you'll do is to create a Rich Condition by spraying a little carburator spray into the engine and observing the oxygen sensors reaction on your scan tool.

If the oxygen sensors (O2S11 and O2S21) are functioning properly, they should immediately react and report 0.800 to 0.900 Volts on your scan tool display screen (in Live Data mode).

If the oxygen sensors are not working properly, meaning they are bad, then they won't respond to the rich condition that you're inducing with the carburetor spray.

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

  1. 1

    Start your 4.0L Ford vehicle and let it idle for about 15 minutes. This will activate the O2 sensors.

    In the meantime, connect the scan tool to your vehicle and get to its Live Data mode (Don't have a scan tool? Need a scan tool? Check out my recommendation: Actron CP9580 Scan Tool).

  2. 2

    In Live Data mode, on your scan tool, scroll down to the PIDs that are labeled O2S11 and O2S21. These are the PIDs that'll show you what voltage values the O2 sensors are producing and sending the PCM.

STEP 3 Continued in the next page...