TEST 1: Inducing A Rich Air/Fuel Mixture

Inducing A Rich Condition. How To Test The Oxygen Sensor (Jeep 4.0L)

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

This can be done by spraying some carburetor cleaner down the throttle with the engine running or my preferred method: down a vacuum hose.

Once the carb spray gets burned in the engine, the air/fuel mixture will instantly go Rich and the O2 sensor should respond by producing its maximum voltage (between 0.800 and 0.900 Volts +). This voltage spike will be recorded 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.

  2. 2

    Connect your scan tool to your Jeep's diagnostic link connector (DLC) and go to it's live data function.

    Don't have a scan tool? Need a scan tool? Check out my recommendation: Actron CP9580 Scan Tool Review.

  3. 3

    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.

  4. 4

    The O2S11 PID voltage numbers should be moving between 0.100 and 0.900 Volts constantly if the engine has been idling for 15 minutes.

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

  5. 5

    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 body 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.

  6. 6

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

  7. 7

    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:

CASE 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 can react and report a rich air/fuel mixture. Now, we need to make sure that it can react to and report a lean air/fuel mixture. For this test go to: TEST 2: Inducing a Lean Air/Fuel Mixture.

CASE 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.

TEST 2: Inducing A Lean Air/Fuel Mixture

Inducing A Lean Air/Fuel Mixture To Test The Front Oxygen Sensor's Responce (4.0L Jeep)

Since you've verified that the oxygen sensor can react to a rich air/fuel mixture and report it, in this section we will induce a lean air/fuel mixture. We can do this easily by:

  1. Disconnecting a large vacuum hose.

Once this large vacuum hose is disconnected (with the engine running), the air/fuel mixture will instantly become lean. This will cause the oxygen sensor to produce its minimum voltage (between 0.100 to 0.200 Volts). We'll be able to see this low voltage reading on the scan tool's display.

The hose we'll disconnect is the one that connects the intake manifold to the vacuum brake booster. The photo above points to this vacuum hose.

Alright, here are the test steps:

  1. 1

    Crank and start your Jeep's engine. Let the engine run for about 15 minutes.

  2. 2

    Connect your scan tool to your Jeep's diagnostic link connector (DLC) and go to it's live data function.

  3. 3

    Now, scroll down to the PID labeled: O2S11.

  4. 4

    Disconnect vacuum hose with the engine running as you watch the reading of the O2S11 PID on your scan tool's display.

    If the engine is stalls, simply restart it and start from step 1 again (and disconnect the hose just a little this time).

  5. 5

    As you are letting air enter the brake booster's vacuum hose, the voltage reading of the O2S11 PID should decrease to about 0.100 to 0.200 Volts.

    As long as the vacuum hose is slightly disconnected and letting ambient air into the intake manifold, the voltage should remain at about 0.100 to 0.200 Volts.

  6. 6

    Reconnect the brake booster's hose and stop the vacuum leak. The voltage reading should now start to move up and down between 0.100 to 0.900 Volts.

Let's take a look at what your test results mean:

CASE 1: The voltage dropped to 0.100 Volt as you opened the brake booster's vacuum hose. This tells you that the oxygen sensor is working well since it can react to a lean air/fuel mixture and report it.

Now, if the oxygen sensor passed the test in TEST 1 and passed this test (TEST 2), then you can conclude with certainty that the oxygen sensor is working properly and it doesn't need to be replaced. If the computer is still accusing it of being defective, take a look at this section: Oxygen Sensor Codes Keep Coming Back.

CASE 2: The voltage DID NOT drop to 0.100 Volt as you opened the brake booster's vacuum hose. This test result tells you that the oxygen sensor is defective since it can not react to a lean air/fuel mixture. Replacing the O2 sensor will solve the issue.

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.

More 4.0L Jeep Diagnostic Tutorials

I've written quite a few 4.0L ‘how to’ tutorials that may help you troubleshoot the issues on your Jeep vehicle. You can find the complete list at:

  1. Jeep 4.0L Index Of Articles.

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

  1. How To Test The Fuel Pump In 2 Tests (4.0L Jeep).
  2. How To Test Engine Compression (Jeep 4.0L).
  3. How To Troubleshoot A Bad Fuel Injector (Jeep 4.0L).
  4. How To Test For A Blown Head Gasket (Jeep 4.0L).
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