This tutorial will help you test the front oxygen sensor heater element and expertly diagnose trouble code P0135: O2 Sensor 1/1 Heater Failure.

You'll be able to check that the upstream O2 sensor is getting power and ground. You'll also learn how to check the O2 sensor's internal heater element with a multimeter resistance test.

In case you're wondering, the front O2 sensor is known by several names: upstream O2 sensor, pre-catalytic converter O2 sensor, oxygen sensor Bank 1 Sensor 1, or O2 sensor 1/1.

Here are the contents of this tutorial:

  1. Circuit Descriptions of the Upstream Oxygen Sensor.
  2. TEST 1: Verifying the Heater Element is Getting Power.
  3. TEST 2: Verifying the Heater Element is Getting Ground.
  4. TEST 3: Testing the Heater Element's Resistance.
  5. Where to Buy the Oxygen Sensor and Save Some $$$.

The 1998 thru' 2000 2.5L Dodge Stratus (2.5L Chrysler Cirrus) uses a different oxygen sensor (the connector is round). To test this upstream oxygen sensor, go here: Oxygen Sensor Heater Test -P0135 (1998-2000 2.5L Stratus/Cirrus).

Circuit Descriptions of the Upstream Oxygen Sensor

As you're already aware, there are 4 wires coming out the upstream O2 sensor on your 1995, 1996, 1997 2.5L V6 Dodge Stratus (Cirrus).

2 of these 4 wires feed power and ground to the heater element. The other 2 are the oxygen signal and oxygen sensor ground wires.

The terminals on the oxygen sensor itself are round male terminals. The terminals on the oxygen sensor engine wiring harness connector are female.

Below, you'll find the color of the wires of the engine wiring harness oxygen sensor connector:

Upstream Oxygen Sensor Pinout
(1995-1997 2.5L Stratus, Cirrus)
Pin Wire Color Description
1 BLK/LT BLU Sensor Ground
2 BLK/DK GRN Oxygen Sensor Signal
3 DK GRN/ORG Heater Element Power
4 BLK Heater Element Ground

TEST 1: Verifying the Heater Element is Getting Power

The upstream oxygen sensor (O2 sensor 1/1) gets power from the dark green with orange stripe (DK GRN/ORG) wire of the O2 sensor engine wiring harness connector.

The DK GRN/ORG wire in turn gets battery power (10 to 12 Volts DC) from the Auto Shutdown (ASD) relay.

To find out if power is present on the DK GRN/ORG wire, we're gonna' do a simple multimeter voltage test.

IMPORTANT: The O2 sensor can get and stay very hot! Perform this test with a completely cold engine. Be careful and take all necessary safety precautions!

OK, this is what you'll need to do:

  1. 1

    Locate the upstream oxygen sensor and disconnect it from its harness connector.

  2. 2

    Locate the DK GRN/ORG wire of the engine wiring harness oxygen sensor connector.

  3. 3

    With your multimeter in Volts DC mode, probe the DK GRN/ORG with the red multimeter lead.

    Ground the black multimeter lead directly on the battery's negative terminal.

  4. 4

    With the Key On Engine Off (KOEO), the DK GRN/ORG wire should have 10 to 12 Volts DC.

Let's take a look at your test results:

CASE 1: Your multimeter confirms that the DK GRN/ORG wire has 10 to 12 Volts DC- So far so good since this test result confirms that the upstream oxygen sensor's heater element is getting power.

The next step is to make check that the BLK wire, of the O2 sensor harness connector, is feeding ground to the heater element. For this test, go to TEST 2: Verifying the Heater Element is Getting Ground.

CASE 2: Your multimeter confirms that the DK GRN/ORG wire DOES NOT have 10 to 12 Volts DC- Re-check that you're testing the correct wire and that the Key is in the RUN position (but don't crank or start the engine) and re-test.

If you still don't see 10 to 12 Volts DC... then this test result tells you that the upstream oxygen (O2) sensor itself IS NOT BAD... since without power, the heater element won't work.

Although it's beyond the scope of this article... the next step is to find out why this battery power is missing using a wiring diagram.