Testing The Front O2 Sensor's Heater (1998, 1999, 2000, 2001 2.5L Ford Ranger And Mazda B2300)

In this tutorial I'll explain how to test the front oxygen sensor (HO2S 11) heater on the 1998, 1999, 2000, or 2001 2.5L Ford Ranger and Mazda B2500 pickup truck.

When a trouble code P0135 pops up and illuminates the check engine light on your 2.5L Ford Ranger, it usually means that the front O2 sensor's heater is fried.

In 8 out of 10 cases, replacing the front O2 sensor (HO2S 11) will solve the problem. In the other cases, the trouble code is lighting up the check engine light because the front O2 sensor's heater isn't getting either power or Ground.

In this tutorial I'll explain how to make sure that the front O2 sensor's heater is getting power and Ground. I'll also explain how to test it's internal resistance to find out if it's truly defective or not.

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

The tutorial for testing the rear O2 sensor (HO2S 11) is here: How To Test The Rear 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 #1 (HO2S 11) Basics

As you've probably already noticed, the front O2 sensor (HO2S 11) has 4 wires sticking out of its electrical connector.

To test the front O2 sensor's internal heater, you need to know which 2 of the 4 wires feed it with power and Ground.

In the table below, you'll find a short description of each wire. Also, if you need the wiring diagram for the front O2 sensor, you can find it here: Oxygen Sensor Circuit Diagram (1998-2001 2.5L Ford Ranger).

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

IMPORTANT: The front oxygen sensor (HO2S 11) can reach some pretty extreme hot temperatures. And it also stays extremely hot even after the engine has been shut down. For this reason perform all of the tests in this tutorial with a completely cold engine!

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 Front O2 Sensor's Heater (1998, 1999, 2000, 2001 2.5L Ford Ranger And Mazda B2300)

The first test that we're gonna' do is to make sure that the front O2 sensor's heater is getting power.

This power comes from fuse #13 (15 A) of the under-hood fuse box on the 1998 through 2000 2.5 ford Ranger. On the 2001 Ford Ranger, this power comes from fuse #41 (20 A) of the under-hood fuse box.

To check for power, we're gonna' disconnect the front O2 sensor from its engine wiring harness connector and test the light blue with orange stripe (LT BLU/ORG) wire with the key on engine off (KOEO).

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 (see photo above).

These are the test steps:

  1. 1

    Disconnect the front 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: Power is present in the light blue with orange stripe wire. This is the correct test result.

Now that you have confirmed that the front O2 sensor's heater is getting power, the next stop is to make sure it's getting Ground on the red with white stripe (RED/WHT) wire. For this test go to: TEST 2: Making Sure That Ground Is Present.

CASE 2: Power IS NOT present in the light blue with orange stripe wire. The front O2 sensor's heater is not going to activate if it's not getting power. This in turn will cause the fuel injection computer to set a trouble code P0135 that will illuminate the check engine light.

This lack of power is usually caused by one of two things: The fuse that feeds this power is blown or there's an open in the LT BLU/ORG wire between the O2 sensor connector and the under hood fuse box.

Your next step is to check fuse #13 of the under hood fuse box (1998-2001 2.5L Ranger) or fuse #41 of the under hood fuse box (2001 2.5L ford Ranger).