P0141 Trouble Code Diagnostic Tests (1990-1997 2.3L Ford)

When the heater element of the downstream oxygen sensor fails on your 2.3L Ford Ranger (Mazda B2300), a P0141 trouble code will light up the check engine light.

The cool thing is that verifying that the O2 sensor's heater element (to see if it has truly fried) isn't hard. In this tutorial I'll show you how to test it. With the result from this test you'll find out if you need to replace the downstream oxygen sensor or not.

Here are the contents of this tutorial:

  1. Circuit Descriptions of the Upstream Oxygen Sensors.
  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 2.3L Ford Tutorials.

En Español You can find this tutorial in Spanish here: Calentador del Sensor de Oxígeno Trasero -P0141 (2.3L Ford) (at: autotecnico-online.com).

Circuit Descriptions of
the Upstream Oxygen Sensors

P0141 Trouble Code Diagnostic Tests (1990-1997 2.3L Ford)

The downstream oxygen sensor on your 2.3L equipped Ford (or Mazda B2300) goes by several different names, among them:

  1. O2 sensor after the catalytic converter.
  2. Rear oxygen (O2) sensor.
  3. Bank 1 Sensor 2.
  4. O2S12.
  5. HO2S (Heated Oxygen Sensor).
  6. Post-catalytic converter oxygen (O2) sensor.

As you're already aware, the rear oxygen sensor has 4 wires coming out of its connector.

2 of these 4 wires provide battery power and ground to the heater element. 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 P0141 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.

Downstream O2 Sensor Circuits
(1995-1997 2.3L Ford Ranger and Mazda B2300)
Pin Wire Color Description
1 Black w/ White stripe (BLK/WHT) Heater Element Ground
2 Light Blue w/ Orange stripe (LT BLU/ORG) Heater Element 12 Volts
3 Red w/ Light Green stripe (RED/LT GRN) HO2S High Signal
4 Orange (ORG) HO2S Low Signal

TEST 1: Verifying the Heater Element is Getting Power

P0141 Trouble Code Diagnostic Tests (1990-1997 2.3L Ford)

Since the heater element of the rear O2 sensor needs battery voltage (and ground) to work, the first thing we'll do is check that it indeed is being fed this battery voltage.

The female terminal labeled with the number 2 of the rear O2 sensor engine wiring harness conector is the one that supplies this power (see the photo above).

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 downstream oxygen sensor and disconnect it from its harness connector.

  2. 2

    Locate the wire that connects to terminal number 2 of the engine wiring harness oxygen sensor connector.

    The wire that connects to terminal number 2 is the Light Blue w/ Orange stripe wire.

  3. 3

    With your multimeter in Volts DC mode, probe the wire that connects to terminal number 2 of the O2 harness connector (see illustration in the image viewer).

    Ground the black multimeter 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 battery voltage (10-12 volts)- This tells you that power is being fed to the rear O2 sensor's heater element.

Now you need to check that terminal number 1, of the O2 sensor harness connector, is feeding ground to the rear O2 sensor's 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- Double check your multimeter test lead connections and repeat the test. If your multimeter still doesn't show 10 to 12 volts..

... Then you have found a problem that needs to be resolved for the O2 sensor's heater to 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.