Rear Oxygen Sensor Heater Test -P0141 (1999-2000 4.0L Jeep)

Testing a trouble code P0141: 1/2 O2 Sensor Heater Failure is not that hard and in this tutorial I'm gonna' show you how to do it in 3 simple tests. You don't need any expensive diagnostic equipment for the tests. All you'll need is a simple multimeter.

If you've been wondering what a trouble code P0141 is trying to tell you... it usually indicates that the rear oxygen sensor's internal heater element has fried on your 1999-2000 4.0L Jeep Grand Cherokee.

IMPORTANT: This tutorial only covers the Federal Emissions equipped 4.0L Jeep Grand Cherokee that's equipped with only 2 oxygen sensors. In contrast, the California Emissions equipped 4.0L Grand Cherokee has 4 oxygen sensors.

NOTE: This oxygen sensor is known by several different names:

  1. Downstream Heated Oxygen (O2) Sensor.
  2. HO2S Sensor 2.
  3. Rear Oxygen (O2) Sensor.
  4. Oxygen Sensor Bank 1 Sensor 2.
  5. Oxygen Sensor After the Catalytic Converter.
  6. Post-Catalytic Converter O2 Sensor.

Here are the contents of this tutorial:

  1. Circuit Descriptions of the Downstream 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 $$$.
  6. More Jeep 4.0L Test Tutorials.

NOTE: The 1996-1998 4.0L Jeep Grand Cherokee's rear O2 sensor is tested in the exact same way as the 1999-2000 4.0L Grand Cherokee. The only difference is the colors of the wires of the engine wiring harness rear O2 sensor connector. For the 1996-1998 P0141 test tutorial, go here: Rear O2 Sensor Heater Test -P0141 (1996-1998 4.0L Grand Cherokee).

Circuit Descriptions of the Downstream Oxygen Sensor

Rear Oxygen Sensor Heater Test -P0141 (1999-2000 4.0L Jeep)

The oxygen sensor has to be at a certain temperature (600°F +) for it to activate and start sensing the oxygen content of the exhaust stream it's exposed to.

When you first start up your Jeep, the engine's exhaust isn't hot enough to get the O2 sensor activated fast enough. Also, the exhaust temperature cools below 600°F when the engine is idling. These gaps, in the oxygen sensor's operation, result in your engine polluting more.

The only way to keep the oxygen working under any engine operating condition is with an internal heater. The heater only requires to things to activate and these are 12 Volts and ground.

So, to test the oxygen sensor's heater, we need to know what wires feed it with power and ground (since it's a 4-wire oxygen sensor).

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

Downstream Oxygen Sensor (HO2S 12) Pinout
(1999-2000 Jeep 4.0L)
Pin Wire Color Description
1 DK GRN/ORG or VIO/WHT Heater Power (+)
2 BLK Heater Ground (-)
3 BLK/LT BLU O2 Signal Ground
4 BLK/DK GRN O2 Signal

TEST 1: Verifying the Heater Element is Getting Power

Rear Oxygen Sensor Heater Test -P0141 (1999-2000 4.0L Jeep)

As mentioned before, the oxygen sensor's internal heater needs battery power and ground to activate and start heating the sensor. So, the very first thing we're gonna' do is check that the rear O2 sensor is being fed with battery power when you turn the key to it's On position.

The wire that feeds this battery power is going to be a dark green with orange stripe (DK GRN/ORG) wire or a violet with white stripe (VIO/WHT) wire of the engine wiring harness rear oxygen sensor connector.

We'll do a simple multimeter voltage test to verify that these 12 Volts are present in the DK GRN/ORG wire.

CAUTION: The oxygen sensor, the catalytic converter, and the exhaust pipe get and stay very hot even after the engine is off! Perform this test with a completely cold engine. Be careful and take all necessary safety precautions! If you raise your vehicle with a jack, place it on jack stands!

IMPORTANT: The pinout in the illustration above is of the connector on the oxygen sensor itself (which has female terminals). To check for power, you need to test the DK GRN/ORG wire of the engine wiring harness sensor connector (which has male terminals).

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

  1. 1

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

  2. 2

    Locate the DK GRN/ORG (or VIO/WHT) 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 RED wire should have 10 to 12 Volts DC.

Let's take a look at your test results:

CASE 1: The test confirms that the DK GRN/ORG (or VIO/WHT) wire has 10 to 12 Volts DC- Good, since this confirms that the rear oxygen sensor's heater element is getting power.

The next step is to make check that the black (BLK) wire, of the O2 sensor engine wiring 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: The test confirms that the DK GRN/ORG (or VIO/WHT) 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 downstream 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.