Continued from the previous page.

  1. 3

    Now, take a look at the voltage readings for O2S11.

    If the engine has been running for about 15 minutes, and IF the 02 sensors are OK, you should see the voltages moving between 0.200 Volts and 0.900 Volts.

    If the voltages are not moving between 0.100 and 0.900 Volts, don't worry about it just yet, continue on to the next step.

  2. 4

    With the engine running, spray a little carburetor cleaner into a vacuum hose (that has engine vacuum) while you observe your scan tool's display screen.

    You're not going to be able to spray carb cleaner into the throttle as the engine is running, because if you were to do this, the engine will die as soon as you disconnect the air duct from it (to spray into it).

    If you spray too much, the engine will stall. If this happens to you, just restart the engine and repeat the step and spray less carb cleaner spray.

  3. 5

    As you spray some short burst of carb cleaner into the vacuum hose, you should see the voltage numbers of O2S11 and O2S21 immediately spike to 0.800 to 0.900 Volts. And as long as you're spraying, these voltage number should stay there.

  4. 6

    When you stop spraying, the O2 sensor values should come down and within a few seconds, they should start oscillating between 0.100 Volts to 0.900 Volts. If they don't, don't worry about it yet.

OK, the test is done, let's take a look at what your test results mean:

TEST RESULT 1: O2S11 and O2S21 voltage numbers spiked to 0.900 Volts when you sprayed carb spray into the vacuum hose. This is the normal (and correct) oxygen sensor reaction and tells you that both oxygen sensors are OK at this point in time.

This test result also tells you that the oxygen sensors don't need to be replaced, since what's causing the PCM to think they're fried is something else.

For more info on this, go to the section: Oxygen Sensor Codes Keep Coming Back.

TEST RESULT 2: O2S11 voltage numbers DID NOT spike to 0.900 Volts when you sprayed carb spray into the vacuum hose. This confirms that Bank 1 Oxygen Sensor 1 is no longer working. You can replace the oxygen sensor.

The reason why is that if the sensor were working correctly, then it would've reacted to the induced rich condition you created (by spraying carburetor spray into the vacuum hose). Since it didn't react, in other words its voltage output did not spike up, then you and I can conclude that it's defective and needs to be replaced.

TEST RESULT 3: O2S21 voltage numbers DID NOT spike to 0.900 Volts when you sprayed carb spray into the vacuum hose. This confirms that Bank 2 Oxygen Sensor 1 is no longer working. You can replace the oxygen sensor.

The reason why is that if the sensor were working correctly, then it would've reacted to the induced rich condition you created (by spraying carburetor spray into the vacuum hose). Since it didn't react, in other words its voltage output did not spike up, then you and I can conclude that it's defective and needs to be replaced.

Oxygen Sensor Codes Keep Coming Back

One of the most common problems I get emailed about is that the oxygen sensor code keeps coming back (and lighting up the check engine light) even after replacing the sensor or sensors with new ones.

When this happens, it's usually due to either a Rich condition or Lean condition that's being caused by some other component on the engine.

In plain English this means that something else on the engine has gone bad and is making the computer over-inject or under-inject fuel. Since the computer is not that smart, it usually just ends up accusing the sensors as bad, instead of the actual condition causing the problem, when all they're doing is just reporting the rich or lean condition.

What sucks about this, is that whatever is causing the Rich or Lean condition is not present all of the time (specially when you're conducting the tests). This happens quite a bit. The strategy here is to wait a few days for the condition (that's causing the O2 sensor issues) to get worse. When this occurs, you'll be able to test it and solve it.

The other key, to diagnosing this type of problem, is to think what could be causing the extreme rich or extreme lean condition and then test those components.

Things that could cause an extreme rich condition are:

  • Bad fuel pressure regulator leaking fuel into its vacuum line.
  • A misfire caused by:
    • Extremely worn spark plugs.
    • One or several spark plug wires that are not transmitting spark anymore to the spark plugs.
    • A bad ignition coil (remember the coil on the 4 .0 liter engines feed spark to all 6 cylinders.
  • Problems with the fuel injectors like:
    • A bad fuel injector stuck open.
    • Dirty fuel injectors.

Things that could cause an extreme lean condition are:

  • Manifold intake gaskets that are bad and leading vacuum.
  • Vacuum hoses that are leaking vacuum.
  • Fuel pump that is bad (more specifically a fuel pump that's slowly going bad and not producing enough fuel pressure and volume even though it's able to keep the engine running.

Other things that you need to keep in mind are: the possibility of a short in the wiring between the O2 sensor and the computer or that the computer may be bad (although this is extremely rare).

More Test Articles

I've written quite a few Ford 4.0L 'how to' tutorials that may help you troubleshoot the issues on your Ford pick up, mini-van or SUV. You can find the complete list at: Ford 4.0L Index Of Articles.

Here's a sample of the Ford 4.0L V6 articles you'll find:

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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