TEST 2: Making Sure The Heater Is Getting Ground
The black with white stripe (BLK/WHT) wire of the O2 sensor's engine wiring harness connector is the wire that supplies Ground to the O2 sensor's heater.
We'll do a simple multimeter voltage test to confirm the presence of Ground in the BLK/WHT wire.
This Ground is a chassis Ground and is present at all times (no need to turn the key to the ON position).
If the BLK/WHT wire is providing Ground to the heater, then the next step is to test its resistance in TEST 3.
OK, let's get testing:
Disconnect the O2 sensor from its engine wiring harness connector.
NOTE: The O2 sensor's engine wiring harness connector has male terminals.
Set your multimeter to Volts DC mode.
Connect the red multimeter test lead to the battery positive (+) terminal.
With the black multimeter test lead, probe the male terminal that connects to the BLK/WHT wire.
NOTE: This is the male terminal of the engine wiring harness connector.
Your multimeter should read 10 to 12 Volts DC if the BLK/WHT wire is providing Ground.
Let's take a look at your test results:
CASE 1: The BLK/WHT wire is providing Ground. This is the correct and expected test result and it confirms that the o2 sensor's heater element is getting Ground.
For our last test we need to check the heater's resistance, go to: TEST 3: Testing The Heater Element's Resistance.
CASE 2: The BLK/WHT wire IS NOT providing Ground. 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/WHT 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've checked the basics, the last thing we need to do is make sure that the heater element's resistance is within specification (4 to 6 Ohms).
We'll accomplish this resistance test with the multimeter in Ohms mode.
If the upstream 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 P0135 trouble code.
OK, this is what you need to do:
Locate the O2 sensor's female terminals labeled with the numbers 1 and 2 (in the illustration above).
These female terminals are on the O2 sensor connector itself (not the engine wiring harness O2 connector).
Place your multimeter in Ohms mode.
Measure the resistance across terminals 1 and 2 of the O2 sensor itself with the multimeter test leads.
Your multimeter should register 4 to 6 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 specification. 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:
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 (1993-1995 4.0L Jeep Cherokee)
- How To Do An Engine Compression Test (1993-2001 4.0L Jeep Cherokee)
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