TEST 2: Verifying The Heater Element Is Getting Ground
In TEST 1 you verified that the DK GRN/ORG (or VIO/WHT) wire, of the engine wiring harness rear O2 connector, has power.
Now, you need to make sure that the black (BLK) wire has Ground.
Since this Ground is provided by your 4.0L Jeep's PCM, we're gonna' do a simple and safe multimeter voltage test to check for its presence.
NOTE: The pinout in the illustration above is of the connector on the oxygen sensor itself. To check for power, you need to test the BLK wire of the engine wiring harness sensor connector.
CAUTION: Do not accidentally or intentionally apply battery power to the BLK wire or you will fry your Jeep's PCM. The multimeter voltage test described below is a safe way to test for Ground in this wire.
These are the test steps:
Locate the BLK wire of the O2 sensor's engine 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.
Turn the key to the ON position but don't crank or start the engine.
Place your multimeter in Volts DC mode.
Connect the red multimeter test lead to battery positive (+) post.
Probe the BLK wire of the O2 sensor's harness connector 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: The rear O2 sensor's heater is being fed Ground. This confirms the rear O2 sensor's heater is being fed with Ground.
So far you've confirmed that the downstream O2 sensor'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: The multimeter confirms that the BLK wire IS NOT feeding Ground to the rear O2 sensor's heater. Re-check all of your connections and make sure you're testing the correct terminal.
If your multimeter still doesn't register the 10 to 12 Volts DC, then the most likely cause of this missing Ground is an ‘open’ in the BLK wire between the O2 sensor's harness connector and chassis Ground.
TEST 3: Testing The Heater Element's Resistance
If you've reached this point, you have checked the basics. These basics are:
- You have a P0141 trouble code lighting up the check engine light on your Jeep.
- The DK GRN/ORG (or VIO/WHT) wire is feeding the rear O2 sensor's heater 12 Volts DC (TEST 1).
- The BLK wire is feeding the rear O2 sensor's heater with Ground (TEST 2).
For our last test, and to find out if the rear O2 sensor's internal heater is fried, you need to check it's resistance with your multimeter in Ohms mode.
If the resistance is not within specification, then you can correctly conclude that the rear O2 sensor needs to be replaced to solve the P0141 diagnostic trouble code.
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 female terminals 3 and 4 of the O2 sensor connector itself (not the engine wiring harness O2 connector).
With your multimeter in Ohms mode.
Measure the resistance across terminals 3 and 4 of the O2 sensor itself with the multimeter test leads.
You should see about 4 to 7 Ωs on your multimeter (you'll typically see about 5 Ohms).
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 rear O2 sensor's heater resistance is within specification. This test result tells you that rear oxygen sensor's heater is OK.
CASE 2: Your multimeter showed an open circuit (OL). This confirms that the downstream O2 sensor's heater element is fried. Replacing the downstream O2 sensor with a new one will solve the P0141 trouble code lighting up the check engine light (CEL).
Remember, the rear O2 sensor is bad only if you have:
- Confirmed that the rear O2 sensor's heater element is getting power (TEST 1).
- Confirmed that the rear 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.
Taking all of the above into account you can correctly conclude that the downstream O2 sensor needs to be replaced with a new one.
Location Of The Oxygen Sensors
NOTE: The above illustration is for the 1999-2000 4.0L Jeep with Federal Emissions. Federal Emissions equipped 4.0L Jeep Grand Cherokees have only 2 oxygen sensors. The 1999 model year Jeep 4.0L sold in California has 4 oxygen sensors and the above illustration does not apply to it.
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:
- Rear O2 Sensor Heater Test -P0141 (1996-1998 4.0L Grand Cherokee).
- 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!