Testing The Oxygen Sensor By Inducing A Rich Condition

How To Test The Alternator (Ford 1.9L, 2.0L)

To test the oxygen sensor (O2S11), the very first thing you'll do is to induce a Rich condition.

This can easily be done by spraying a little carburetor cleaner into the engine while it's running. My preferred method is to spray carb spray into a vacuum hose.

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

    Start your vehicle and let it idle for about 15 minutes, since you need a warmed up engine to get the O2 sensor to activate.

    Connect your scan tool 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

    Once you're in Live Data mode, scroll down to the PID that's labeled O2S11. This PID will show you the oxygen sensor voltage activity.

    Look at the photo in the image viewer above to see an example of this.

  1. 3

    What you should see, if the engine has been idling for about 15 minutes, are the voltage numbers of the O2 sensor moving between 0.100 and 0.900 Volts constantly.

    If the voltage value stays fixed, don't worry about this yet, continue 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 bursts of carb cleaner into the vacuum hose, you should see the voltage numbers of O2S11 immediately spike to 0.800 to 0.900 Volts. And as long as you're spraying, these voltage numbers 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 voltage numbers spiked to 0.900 Volts when you sprayed carb spray into the vacuum hose. This tells you that the oxygen sensor is OK at this point in time. It does not need to be replaced, since whatever's causing the PCM to think it's 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 (in other words, spraying the carburetor cleaner had no effect on the oxygen sensor reading your scan tool registered at the beginning of the test) 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.

Oxygen Sensor Codes Keep Coming Back

So you tested the oxygen sensor and they tested good or you have already replace one or both and yet the PCM keeps accusing them as being bad (which also means that the check engine light is back on).

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.

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.

Ford Vehicles:

  • Escort
    • 1995, 1996, 1997, 1998, 1999, 2000, 2001, 2002
  • Focus
    • 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009

Mercury Vehicles:

  • Tracer
    • 1995, 1996, 1997, 1998, 1999