Front Oxygen Sensor Heater Test -P0135 (1996-2000 4.0L Jeep)

A trouble code P0135: O2 Bank 1 Sensor 1 Heater Circuit Performance on your Jeep Cherokee (Grand Cherokee, Wrangler) lets you know that there's a problem with the heater element of the upstream O2 sensor.

This code usually points to a bad upstream oxygen sensor. The cool thing is that testing the heater element of the upstream O2 sensor, to make sure the sensor is bad or not, is easy. In this tutorial I'll show you how in a step-by-step way.

En Español You can find this tutorial in Spanish here: Calentador del Sensor de Oxígeno Delantero -P0135 (1996-2000 4.0L Jeep) (at: autotecnico-online.com).

If you need to troubleshoot the rear O2 sensor (HO2S 1/1) P0141 trouble code, take a look at the following tutorial: Rear Oxygen Sensor Heater Test -P0141 (1996-1999 4.0L Jeep).

Circuit Descriptions Of The Upstream Oxygen Sensors

Front Oxygen Sensor Heater Test -P0135 (1996-2000 4.0L Jeep)

The upstream oxygen sensor on your 4.0L Jeep Cherokee (Grand Cherokee, Wrangler) is known by several different names... among them:

  1. O2 sensor before the catalytic converter.
  2. Front oxygen (O2) sensor.
  3. Bank 1 Sensor 1.
  4. O2S11.
  5. 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.

O2 Bank 1 Sensor 1 Circuits
Pin Description
1 Heater Element 12 Volts (from Auto Shutdown Relay)
2 Heater Element Ground
3 HO2S Low Signal
4 HO2S High Signal

TEST 1: Verifying The Heater Element Is Getting Power

Front Oxygen Sensor Heater Test -P0135 (1996-2000 4.0L Jeep)

We'll start our testing by making sure that your Jeep's upstream O2 sensor's heater element is getting power.

This power comes in the form of 12 volts... which we can easily verify with a multimeter in volts DC mode.

CAUTION: Perform this test on a completely cold engine or you run the risk of getting severely burned by the O2 sensor! 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 wire that connects to terminal number 1 of the engine wiring harness oxygen sensor connector.

  3. 3

    With your multimeter in Volts DC mode, probe the wire that connects to terminal number 1 of the O2 harness connector (see illustration in the image viewer).

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

  4. 4

    With the Key On Engine Off (KOEO), 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 the correct and expected test result and tells you that your 4.0L Jeep's upstream oxygen sensor's heater element is getting power..

The next step is to make check that terminal number 2, 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 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 4.0L Jeep 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.