TEST 2: Checking The Heater Ground
When the upstream O2 sensor on your Dodge Neon or Plymouth Neon gets power, this voltage automatically gets Ground from the black (BLK) wire of the connector (on the engine wiring harness).
And so, our next testing step, in troubleshooting the P0135, is to make sure that the Bank 1 Sensor 1 oxygen sensor is getting Ground in this wire.
This is what you'll need to do:
Disconnect the upstream oxygen sensor (if it isn't already from TEST 1).
Locate the wire that connects to terminal number 3 of the O2 harness connector.
You'll test the wire that is 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 the battery (+) post.
Connect the black multimeter test lead to the wire that connects to terminal number 3 of the engine wiring harness oxygen sensor connector.
With the Key On, Engine Off, this wire should have 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 is the correct and expected test result and confirms that the pre-catalytic oxygen sensor is getting Ground on your Dodge Neon (Plymouth Neon).
Now that we have confirmed that the O2 sensor connector is feeding both power and Ground, the next and final test, is to see if the heater element itself is BAD or not. This is a simple test that requires you to check the heater element's resistance with your multimeter. For this test, go to: TEST 3: O2 Heater Resistance Test.
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 does not indicate the 10 to 12 Volts DC, then the black wire has an ‘open’.
By an ‘open’, 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 1995-1996 Dodge Neon (Plymouth Neon).
TEST 3: O2 Heater Resistance Test
So far you have checked and confirmed that:
One: The upstream oxygen (O2) sensor's heater element is getting power (10 to 12 Volts DC) on the dark green with orange stripe wire (in TEST 1).
Two: In TEST 2, you confirmed that the heater element is getting Ground on the black wire.
Now comes the final test and this is to check the internal resistance of the O2 sensor's heater element itself with your multimeter in Ohms (Ω) mode.
This resistance test will tell you if the heater element is fried or not, and if it's fried then you can now buy a new oxygen sensor, for Dodge or Plymouth Neon, knowing it's gonna' solve the problem.
OK, this is what you need to do:
Disconnect the upstream oxygen sensor from the engine wiring harness connector (if it isn't already from the previous tests).
NOTE: The O2 sensor must be disconnected from the vehicle's connector for this test!
Locate the O2 sensor terminals labeled with the number 3 and number 4 in the photo above.
NOTE: You're gonna' be testing these two terminals on the oxygen sensor itself.
Place your multimeter in Ohms mode.
Probe terminals number 3 and number 4 with the multimeter test leads.
If all is OK, you should see about 5 to 16 Ω (Ohms).
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 is the correct test result and tells you that the Pre-catalytic converter oxygen (O2) sensor on your 2.0L Neon is OK.
Now, since you have a P0135 lighting up your check engine light, I suggest that you erase the trouble code and road test the vehicle. If the diagnostic trouble code does come back, I suggest you replace the O2 sensor since there's a possibility that there's intermittent short-circuit problem in its connector.
This is a very rare condition but I have seen it every now and then.
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.
Replacing the oxygen sensor with a new one will solve the P0135 diagnostic trouble code that is illuminating the check engine light on your Dodge Neon (Plymouth Neon).
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