P0135 Oxygen Sensor 1996-2001 3.9L, 5.2L, 5.9L Dodge Ram

A trouble code P0135: O2 Bank 1 Sensor 1 Heater Circuit Performance indicates that the front (upstream) oxygen sensor’s heater, on your 1996-2001 Dodge Ram pick-up, has failed... but not always.

There’s a good chance that the O2 sensor’s heater might be missing power or ground.

In this tutorial, I’ll show you how to test the upstream oxygen (O2) sensor’s internal heater element to see if it’s fried (and causing the P0135 DTC) or not.

Here are the contents at a quick glance:

  1. Circuit Descriptions Of The Upstream Oxygen Sensors.
  2. TEST 1: Verifying The Heater Is Getting Power.
  3. TEST 2: Checking The Heater Ground.
  4. TEST 3: O2 Heater Resistance Test.
  5. Where To Buy The Oxygen Sensor And Save Some $$$.

Circuit Descriptions Of The Upstream Oxygen Sensors

P0135 Oxygen Sensor 1996-2001 3.9L, 5.2L, 5.9L Dodge Ram

As you’re probably already aware, your Dodge Ram has two oxygen sensors. One’s located before the catalytic converter and the other is located after it.

Although both are tested in the same way, this tutorial focuses on testing the front O2 sensor’s heater element.

You’ve probably also found out by now that the front O2 sensor is known by several names, such as:

  1. O2 sensor before the Catalytic Converter.
  2. Front oxygen (O2) sensor.
  3. Bank 1 Sensor 1.
  4. Pre-catalytic converter oxygen (O2) sensor.

It doesn’t matter what the upstream 02 sensor is called... the fact of the matter is that it has 4 wires coming out of it.

2 of these 4 wires belong the the heater element inside the oxygen sensor. The other 2 are the ones that provide the oxygen content info of the exhaust, to the PCM (Powertrain Control Module = Fuel Injection Computer).

To get to the bottom of the P0135 Trouble Code, we don’t have to test all 4 circuits (wires).. we only have to worry about the 2 wires that supply the heater element with power and ground.

The chart below will help us to identify all of the upstream oxygen sensor wires on your 1996 through 2001 Dodge Ram pick-up.

O2 Bank 1 Sensor 1 Circuits
Pin Wire Color Description
1 Dark Green w/ Orange stripe Heater Element 12 Volts (from Auto Shutdown Relay)
2 Black w/ White stripe Heater Element Ground
3 Black w/ Light Blue stripe HO2S Low Signal
4 Tan w/ White stripe HO2S High Signal

TEST 1: Verifying The Heater Is Getting Power

P0135 Oxygen Sensor 1996-2001 3.9L, 5.2L, 5.9L Dodge Ram

The very first thing we’ll do, to find out what’s causing the P0135 trouble code lighting up the check engine light on your Dodge Ram, is to check that the DK GRN/ORG (dark green/orange) wire is feeding the heater with power.

This power is in the form of 12 volts and are provided by the ASD (auto shut down) relay.

Important: All of the O2 sensor tests should be done with a completely cold engine to avoid the risk of getting severely burned by the O2 sensor! Be careful and take all necessary safety precaution!

OK, this is what you’ll need to do:

  1. 1

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

  2. 2

    Locate the wire that connects to terminal number 1 of the engine wiring harness oxygen sensor connector.

    1. This is usually a Dark Green w/ Orange stripe wire.
  3. 3

    With your multimeter in Volts DC mode:

    1. With the red multimeter lead, probe the wire that connects to terminal number 1 of the O2 harness connector (see illustration in the image viewer).
    2. Ground the black multimeter lead.
  4. 4

    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: Your multimeter recorded 10 to 12 Volts DC- This is good and let’s you know that the oxygen sensor before the Catalytic Converter on your Dodge Ram is getting power from the Auto Shut Down Relay.

The next step is to make sure that the oxygen sensor’s heater element is getting fed ground on terminal number 2 of the O2 sensor’s harness connector. For this test, go to TEST 2.

CASE 2: Your multimeter DID NOT record the indicated Voltage- 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 exonerates the upstream oxygen (O2) sensor on your Dodge Ram as BAD....

Although it’s beyond the scope of this article... the next step is to check the continuity between the terminal number 1 wire of the O2 sensor harness connector and the Auto Shutdown Relay.