TEST 2: Verifying The Heater Element Is Getting Ground
After checking power, the next step is to make sure that the rear O2 sensor's heater is getting Ground.
You can check for Ground in several ways, in this tutorial we'll check for it with a simple multimeter voltage test.
NOTE: The illustration of the connector above is of the connector on the oxygen sensor itself. To check for Ground, you need to test the BLK/TAN wire of the engine wiring harness sensor connector.
These are the test steps:
Disconnect the O2 sensor from its wiring harness connector.
NOTE: Remember, you'll test the wire that's on the engine wiring harness connector side and NOT on the O2 sensor itself.
Place your multimeter in Volts DC mode.
Connect the red multimeter test lead to battery positive (+) post.
Turn the key to the ON position but don't crank or start the engine.
Probe the terminal (of the connector) that connects to the BLK/TAN wire with the black multimeter test lead.
Your multimeter should register 10 to 12 Volts DC.
Let's take a look at your test results:
CASE 1: Your test results show 10 to 12 Volts DC. This tells you that the downstream oxygen sensor's heater element is getting Ground.
So far you've confirmed that the HO2S 12's heater element is getting both power and Ground. The next step is to check the heater element's resistance with your multimeter. For this test, go to: TEST 3: Testing The Heater Element's Resistance.
CASE 2: Your test results DID NOT show 10 to 12 Volts DC. Re-check all of your connections and make sure you're testing the correct terminal.
If you still don't see 10 to 12 Volts DC, then the most likely cause of this missing Ground is an ‘open’ in the BLK/TAN wire between the O2 sensor's engine wiring harness connector and chassis Ground.
TEST 3: Testing The Heater Element's Resistance
Now that you've confirmed that the rear O2 sensor is getting power and Ground, the last test is to do a multimeter resistance test.
What we're looking for is for the rear O2 sensor's heater element's resistance to be within it's factory resistance.
If it is not within specification, then we can conclude that the downstream sensor is fried and the cause of the P0141: 1/2 O2 Sensor Heater Failure trouble code lighting up the check engine light.
NOTE: Just a reminder that the downstream oxygen sensor has to be completely cold before proceeding with this test since the manual calls for the O2 sensor to be at room temperature for the resistance test.
OK, this is what you need to do:
Locate the O2 sensor terminals number 3 and number 4 of the O2 sensor connector itself (not the engine wiring harness O2 connector).
Place your multimeter in Ohms mode
Measure the resistance across terminals number 3 and number 4 of the O2 sensor itself.
If all is OK, you should see about 4 to 7 Ωs on your multimeter.
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 heater's resistance is within factory specification. This test result tells you that the rear oxygen sensor's heater is OK.
CASE 2: Your multimeter showed an open circuit (OL). This confirms that the rear O2 sensor's heater element is fried. Replacing the rear O2 sensor with a new one will solve the P0141 trouble code lighting up the check engine light (CEL).
You can correctly conclude that the sensor is bad and needs to be replaced if you have:
- Confirmed that the downstream O2 sensor's heater element is getting power (TEST 1).
- Confirmed that the downstream O2 sensor's heater element is getting Ground (TEST 2).
- In this test you have confirmed that the heater element's resistance is out of specification.
Location Of The Oxygen Sensors
NOTE: The above illustration is for the 1996-1998 4.0L Jeep Grand Cherokee.
More Jeep 4.0L Test Tutorials
If this tutorial was helpful/informative, you can find a complete list of tutorials here: Jeep 4.0L Index Of Articles.
Here's a sample of the tutorials you'll find there:
- How To Test Trouble Code P0135 (1996-2000 Jeep 4.0L).
- How To Test The MAP Sensor (1997-2003 Jeep 4.0L).
- How To Do A Cylinder Balance Test (Jeep 4.0L).
- Jeep PWM Fan Relay Test Troubleshooting An Overheating Condition.
- How To Test The Crankshaft Position Sensor (1994-1996 4.0L Jeep).
If this info saved the day, buy me a beer!