How To Test The O2 Sensor Heater (1993-1995 4.0L Cherokee)

TEST 2: Making Sure The Heater Is Getting Ground

The wire that supplies the oxygen sensor's heater with Ground is the black with tan stripe (BLK/TAN) wire of the O2 sensor's engine wiring harness connector.

To test for the presence of Ground in the BLK/TAN wire, we're gonna' do a simple multimeter voltage test.

This Ground is a chassis Ground and is present at all times (no need to turn the key to the ON position).

If Ground is present in the BLK/TAN wire, then the next step is to test the heater's resistance in TEST 3.

OK, let's get testing:

1. 1

   Disconnect the O2 sensor from its engine wiring harness connector.
   
   NOTE: The O2 sensor's engine wiring harness connector has female terminals.

2. 2

   Set your multimeter to Volts DC mode.

3. 3

   Connect the red multimeter test lead to the battery postive (+) terminal.

4. 4

   With the black multimeter test lead, gently probe the female terminal that connects to the BLK/TAN wire.
   
   NOTE: This is the female terminal of the engine wiring harness connector.

5. 5

   Your multimeter should read 10 to 12 Volts DC.

Let's take a look at your test results:

CASE 1: Your multimeter registered 10 to 12 Volts DC. This is the correct and expected test result and it lets you know that the heater element is getting Ground.

The next and last step is to check the O2 sensor heater element's resistance. For this test go to: TEST 3: Testing The Heater Element's Resistance.

CASE 2: The multimeter DID NOT register 10 to 12 Volts. Without Ground the O2 sensor's heater will not function.

The most likely cause of this missing Ground is an open-circuit problem in the BLK/TAN wire. Between the O2 sensor connector and the engine.

Your next step is to repair the Ground wire and repeat the test.

TEST 3: Testing The Heater Element's Resistance

Now that you have confirmed that the O2 sensor's heater is being fed with 12 Volts and Ground, we're going to check that the resistance of the heater element is within specification (5 to 7 Ohms).

This is a pretty simple test and it's done with your multimeter in Ohms mode.

If the O2 sensor's heater element is not within specification, then you can confidently replace the O2 sensor knowing that it's going to solve the oxygen sensor trouble code.

OK, this is what you need to do:

1. 1

   Locate the O2 sensor's male terminals labeled with the numbers 3 and 4 (in the illustration above).
   
   These male terminals are on the O2 sensor connector itself (not the engine wiring harness O2 connector).

2. 2

   Place your multimeter in Ohms mode.

3. 3

   Measure the resistance across terminals 3 and 4 of the O2 sensor itself with the multimeter test leads.

4. 4

   Your multimeter should register 5 to 7 Ωs.
   
   If the heater element is fried, your multimeter will show an open (usually indicated by the letters OL) or a number over 10 K Ωs.

Let's take a look at your test results:

CASE 1: The O2 sensor's heater resistance is within spec. This is the correct and expected test result.

Since the O2 sensor's heater element is within specification, you can now conclude that the O2 sensor itself is not the cause of the oxygen sensor trouble code lighting up your check engine light.

CASE 2: Your multimeter showed an open circuit (OL). This confirms that the O2 sensor's heater element is fried.

To be a bit more specific: you can conclude the O2 sensor's heater element is defective if you have:

• Confirmed that the O2 sensor's heater element is getting 12 Volts (TEST 1).
• Confirmed that the O2 sensor's heater element is getting Ground (TEST 2).
• In this test, you have confirmed that the heater element's resistance IS NOT within specification.

More 4.0L Jeep Cherokee Tutorials

You can find a complete list of diagnostic tutorials and wiring diagrams for the 4.0L Jeep Cherokee here:

• Jeep 4.0L Index of Articles

Here's a sample of the tutorials you'll find in the index:

• Ignition System Wiring Diagram (1993-1995 4.0L Jeep Cherokee)
• How To Test For A Blown Head Gasket (1993-2001 4.0L Jeep Cherokee)
• How To Test The Crankshaft Position Sensor Jeep 4.0L (1993-1995 4.0L Jeep Cherokee)
• How To Do An Engine Compression Test (1993-2001 4.0L Jeep Cherokee)