Rear O2 Sensor Heater Test -P0141 (1996-1998 4.0L Grand Cherokee)

A trouble code P0141: 1/2 O2 Sensor Heater Failure signals a problem with the internal heater of the downstream oxygen sensor. Thankfully, testing it isn't difficult at all.

In this tutorial, I'll show you the 3 multimeter tests you'll need to perform to diagnose the right rear oxygen sensor.

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

1. Rear Heated Oxygen (O2) Sensor.
2. HO2S 12.
3. Downstream Oxygen (O2) Sensor.
4. Oxygen Sensor Bank 1 Sensor 2.

If you need to troubleshoot the front O2 sensor (HO2S 1/1), take a look at the following tutorial:

• How To Test Trouble Code P0135 (1996-2000 Jeep 4.0L).

For the 1999-2000 4.0L Grand Cherokee P0141 tests, go to: Rear O2 Sensor Heater Test -P0141 (1999-2000 4.0L Grand Cherokee).

Circuit Descriptions Of The Downstream Oxygen Sensor

As you're probably already aware, an oxygen sensor needs to get hot (600°F +) to activate. As hot as the exhaust stream the O2 sensor is exposed to is, at engine start-up it isn't hot enough to get the O2 sensor to its operating temp fast enough. Also, when the engine idles, the exhaust stream doesn't stay hot enough to maintain the O2 sensor functioning.

This is where the oxygen sensor's internal heater saves the day since, at engine start, it gets the O2 sensor to reach its operating temperature fast. It also maintains the O2 sensor at its operating temperature under all engine operating conditions. This means that your Jeep is polluting less and maximizing gas mileage through out the entire time the engine is running.

Since the rear O2 sensor is heater equipped, you'll see 4 wires sticking out it. 2 wires are for actual oxygen sensing part of the sensor assembly. The other 2 are to supply the heater with power and Ground.

NOTE: You can find the location of the right rear oxygen sensor (HO2S 12) here: Location Of The Oxygen Sensors.

Below, you'll find the color of the wires of the engine wiring harness oxygen sensor connector for sensor HO2S 12:

HO2S 1/2 Pinout 1996-1998 4.0L Jeep 4.0L Grand Cherokee
Pin	Wire Color	Description
1	DK GRN/ORG	Heater Power (+)
2	BLK/TAN	Heater Ground (-)
3	BLK/LT BLU	O2 Signal Ground
4	BLK/PNK	O2 Signal

Where To Buy The Oxygen Sensor And Save

If you find, after testing the downstream oxygen sensor that its heater element is fried, take a look at the links below. I think they'll save you some bucks:

NOTE: If you're not sure if the above O2 sensor fit your particular 4.0L Jeep don't worry, once you get to the site, they'll make sure the sensor is the right one, if not, they'll find you the right one.

TEST 1: Verifying The Heater Element Is Getting Power

Making sure that the oxygen sensor's heater is getting power is the first thing that we're gonna' do to find out if it's bad (or not).

Checking for power is a pretty easy affair since we can do a simple multimeter voltage test.

CAUTION: Start all of your tests with a completely cold engine. This will ensure that the O2 sensor is completely cold. Be careful and take all necessary safety precautions because the O2 sensor can get and stay extremely hot for a long time after engine shut down! Also, if you raise your vehicle with a jack, place it on jack stands.

IMPORTANT: The illustration of the connector above is of the connector on the oxygen sensor itself. To check for power, you need to test the DK GRN/ORG wire of the engine wiring harness sensor connector.

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

1. 1

   Locate the rear oxygen sensor and disconnect it from its engine wiring harness connector.

2. 2

   Place your multimeter in Volts DC mode.

3. 3

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

4. 4

   Turn the key to the ON position but don't crank or start the engine.

5. 5

   Probe the terminal that connects to the DK GRN/ORG wire with the red multimeter test lead.

6. 6

   Your multimeter should register 10 to 12 Volts DC.

Let's take a look at your test results:

CASE 1: Your test result shows that the rear O2 sensor's heater is getting power- This confirms that the rear oxygen sensor's heater element is getting power.

The next step is to make check that the BLK/TAN wire, of HO2S 12 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: Your test result shows that the rear O2 sensor's heater IS NOT getting power. 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 your multimeter still does not register 10 to 12 Volts DC, then you can conclude that HO2S 12 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.