Testing the Oxygen Sensors on Your Ford 4.0L

How To Test the Oxygen Sensor (Ford 4.0L)

Testing the upstream oxygen sensors (O2S11 and O2S21), on your OBD II equipped 4.0L Ford Aerostar, Explorer, Ranger, or Mountaineer, can be accomplished in a few easy steps and in this article I'll show you how.

As you might already be aware, if you've done some research... quite a few things can cause the PCM (Powertrain Control Module = fuel injection computer) to think that the oxygen sensor or sensors have failed when they haven't. So testing them before replacing them is a good idea.

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

  1. Important Suggestions and Tips.
  2. Symptoms of a BAD Oxygen Sensor.
  3. Oxygen Sensor Trouble Code Basics.
  4. Oxygen Sensor Basics.
  5. Where are O2S11 and O2S21 Located?
  6. Oxygen Sensor Test 1: Inducing a Rich Condition.
  7. Oxygen Sensor Codes Keep Coming Back.
  8. More Test Articles.

Important Suggestions and Tips

TIP 1: To test the O2 sensors, with the test info in this article, you'll need a scan tool that has Live Data capability.

You don't need an expensive $2K to $5K (US) scan tool to do it. I have written this article so that you can accomplish the O2 sensor tests 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 test info in this article will show you how to test the oxygen sensors in action and on the car... no need to remove them.

Symptoms of a BAD Oxygen Sensor

A failed or failing upstream oxygen sensor does not cause sever drive-ability issues. For the most part, your 4.0L equipped Ford vehicle will run pretty good. This doesn't mean you won't see or feel some symptoms. Here are the most common symptoms:

  1. The Check Engine Light (CEL) will be illuminated and one or several of the following Diagnostic Trouble Codes will be stored in the PCM's memory:
    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).
  2. BAD Gas Mileage.
  3. Won't pass State Mandated Emission testing.

Oxygen Sensor Trouble Code Basics

If you've been wondering exactly what the heck each Diagnostic Trouble Code means in plain English, this is the section for you:

P0131: Upstream Heated Oxygen Sensor (HO2S11) Circuit Out Of Range Low voltage (Bank 1).

  1. This DTC is telling you that the PCM is seeing the upstream oxygen sensor, for Bank 1, produce a negative voltage number. This negative voltage number is usually between 0 and -1 Volt.

P0133: Upstream Heated Oxygen Sensor (HO2S11) Circuit Slow Response (Bank 1).

  1. This code results when the upstream O2 sensor for Bank 1 is responding too slowly to the changes in the air/fuel mixture the PCM is commanding. In other words, the O2 sensor is getting a bit lazy.

P0151: Upstream Heated Oxygen Sensor (HO2S21) Circuit Out Of Range Low voltage (Bank 2).

  1. This DTC is telling you that the PCM is seeing the upstream oxygen sensor, for Bank 2, produce a negative voltage number. This negative voltage number is usually between 0 and -1 Volt.

P0153: Upstream Heated Oxygen Sensor (HO2S11) Circuit Slow Response (Bank 2).

  1. This code results when the upstream O2 sensor for Bank 2 is responding too slowly to the changes in the air/fuel mixture the PCM is commanding. In other words, the O2 sensor is getting a bit lazy.

The oxygen sensor test, in this article, will help you to diagnose any of the above Diagnostic Trouble Codes.

Ford Vehicles:

  • Aerostar 4.0L
    • 1995, 1996, 1997
  • Explorer 4.0L
    • 1995, 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003
  • Ranger 4.0L
    • 1995, 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003

Mercury Vehicles:

  • Mountaineer 4.0L
    • 1997, 1998, 1999, 2000, 2001, 2002, 2003

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Rodney Dangerfield

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