Testing The Rear O2 Sensor's Heater (1998, 1999, 2000, 2001 2.5L Ford Ranger And Mazda B2500)

Testing the heater element of the rear oxygen sensor (HO2S 12) is not difficult on the 1998, 1999, 2000, and 2001 2.5L Ford Ranger or Mazda B2500.

In this tutorial I'll explain how to test it to find out if its bad or not in 3 easy tests.

You can find this tutorial in Spanish here: Cómo Probar El Calentador Del Sensor De Oxígeno Trasero (1998-2001 2.5L Ford Ranger) (at: autotecnico-online.com).

The tutorial for testing the front O2 sensor (HO2S 11) is here: How To Test The Front O2 Sensor Heater (1998-2001 2.5L Ford Ranger).

NOTE: You can find the oxygen sensor circuit diagram here: Oxygen Sensor Circuit Diagram (1998-2001 2.5L Ford Ranger).

Heated Oxygen Sensor #2 (HO2S 12) Basics

The heater inside the oxygen sensor needs power and Ground to activate. Once activated, the heater heats up the oxygen sensor to about 600 °F.

As you have probably already noticed, there are four wires coming out of the oxygen sensor. Two of those wires feed it with 12 Volts and Ground.

Specifically, the wire that supplies power to the heater element is the light blue with orange stripe (LT BLU/ORG) wire.

The wire that supplies Ground to the heater is the white with black stripe (BLK/WHT) wire of the 4 wire rear O2 sensor's engine wiring harness connector.

Terminal Wire Description
1 Light Blue With Orange Stripe (LT BLU/ORG) 12 Volt Input
2 White With Black Stripe (WHT/BLK) Ground Input
3 Orange (ORG) Sensor Ground Input
4 Red With Light Green Stripe (RED/LT GRN) O2 Signal Output

IMPORTANT: Perform all test with the engine completely cold. This is due to the fact that the O2 sensor can reach extremely high temperatures. Not only that, but it can stay really hot for a while after the engine has been turned off.

TEST 1: Making Sure That Power Is Present

Terminal 1 Should Have 10 To 12 Volts Present With The Key On Engine Off. Testing The Rear O2 Sensor's Heater (1998, 1999, 2000, 2001 2.5L Ford Ranger And Mazda B2500)

To get our rear O2 sensor diagnostic under way, we're going to start off by making sure that the light blue with orange stripe wire is supplying 10 to 12 Volts to the heater element.

The 1998-2000 2.5L Ford Ranger (Mazda B2500), power comes from fuse #13 of the under hood fuse box.

On the 2001 2.5L Ford Ranger (Mazda B2500), power comes from fuse #41 of the under hood fuse box.

CAUTION: Perform all tests with a completely cold engine!

NOTE: This test is done on the engine wiring harness connector. This connector has round female terminals.

Let's get testing:

  1. 1

    Disconnect the rear O2 sensor from its harness connector.

  2. 2

    Locate the light blue with orange wire (LT BLU/ORG) of the engine wiring harness oxygen sensor connector.

  3. 3

    With your multimeter in Volts DC mode, probe the LT BLU/ORG wire with the red multimeter test lead.

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

  4. 4

    With the Key On Engine Off (KOEO), the LT BLU/ORG wire should have 10 to 12 Volts DC.

Let's take a look at your test results:

CASE 1: The rear O2 sensor heater element is getting power. This is the correct test result.

Now that you know power is getting to the heater element, the next step is to make sure that it's getting Ground. For this test go to: TEST 2: Making Sure That Ground Is Present.

CASE 2: The rear O2 sensor heater element IS NOT getting power. Without power, the heater element will not activate.

This lack of power is usually do to a blown fuse. If you're driving a 1998-2000 2.5L Ford Ranger, then check fuse #13 of the under hood fuse box. If you're driving a 2001 2.5L Ford Ranger, then check fuse #41 of the under hood fuse box.

If the fuse is blown, replace it and retest the LT BLU/ORG.