TEST 2: Verifying The Heater Element Is Getting Ground
The next step, in diagnosing the P0135 trouble code lighting up your Jeep's check engine light (CEL), is to make sure that the O2 sensor's heater element is getting Ground.
This Ground is provided by terminal number 2 of the O2 sensor's engine wiring harness connector.
In this test step, we'll check for this Ground, by doing a simple voltage test.
This is what you'll need to do:
Locate the wire that connects to terminal number 2 of the O2 sensor's 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. This connector has male terminals.
Place your multimeter in Volts DC mode.
Connect the red multimeter test lead to battery (+) terminal.
Connect the black lead to terminal number 2 of the O2 sensor's harness connector.
Turn the key ON but don't start the engine.
Your multimeter should register 10 to 12 Volts DC.
Let's take a look at your test results:
CASE 1: The multimeter registered 10 to 12 Volts DC. This test result confirms that the heater element on your 4.0L Jeep Cherokee (Grand Cherokee, Wrangler) is getting Ground.
Now that you know that the upstream O2 sensor is being fed both power and Ground, the next and final test, is to resistance test the heater element itself. For this test, go to: TEST 3: Testing The Heater Element's Resistance.
CASE 2: The multimeter DID NOT register 10 to 12 Volts DC. Re-check all of your connections and make sure you're testing the correct wire.
If your multimeter still doesn't register the 10 to 12 Volts DC, then the wire that connects to terminal number 2 (of the O2 sensor harness connector) has an open-circuit problem.
By an open-circuit problem, I mean that the wire is cut somewhere between it and its Ground point. To confirm this, you can use a jumper wire to Ground this circuit and repeat the test.
Repairing this Ground issue will solve the P0135 issue you're having with your Jeep vehicle.
TEST 3: Testing The Heater Element's Resistance
After having confirmed that the upstream O2 sensor's heater element is getting both power and Ground, the next (and last) test is to check the resistance of the heater element itself.
This is another simple test that you'll accomplish with your multimeter (in Ohms mode).
NOTE: Just a reminder that the upstream 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 1 and number 2 of the O2 sensor connector itself (not the engine wiring harness O2 connector).
With your multimeter in Ohms mode, probe terminals number 1 and number 2 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).
Let's take a look at your test results:
CASE 1: Your multimeter confirmed the indicated resistance. This test result tells you that your Jeep's pre-catalytic converter oxygen (O2) sensor's heater is OK.
CASE 2: Your multimeter showed an open circuit (OL). This confirms that the upstream (Bank 1 Sensor 1) O2 sensor's heater element is fried. Replace the upstream O2 sensor with a new one.
A new upstream oxygen sensor will solve the P0135 diagnostic trouble code that is illuminating the check engine light on your 4.0L Jeep Grand Cherokee (Cherokee, Wrangler).
NOTE: If you'd like to save a few bucks (and sometimes a whole more!) on the oxygen sensor for your Jeep, check out my recommendation on where to buy it in the next page.
More 4.0L Jeep Diagnostic Tutorials
You can find a complete list of Jeep 4.0L test tutorials here: Jeep 4.0L Index Of Articles.
- How To Test The MAP Sensor (1997-2003 Jeep 4.0L).
- How To Test The Starter Motor (Jeep 4.0L).
- Jeep PWM Fan Relay Test Troubleshooting An Overheating Condition.
- How To Test For A Blown Head Gasket (Jeep 4.0L).
- How To Test The Crankshaft Position Sensor (1994-1996 4.0L Jeep).
If this info saved the day, buy me a beer!