P0135 Trouble Code Diagnostic Tests (1990-1997 2.3L Ford)

In this tutorial I'm going to show you how to test the heater element of the upstream oxygen sensor on your 2.3L Ford Ranger (Mustang, Mazda B2300).

No scan tool is required, since the tests I'm gonna' show you are done with just a multimeter.

You'll also be able to troubleshoot trouble code P0135 on 1995 + OBD II equipped 2.3L Ford Rangers.

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 Delantero P0135 (1995-1997 2.3L) (at: autotecnico-online.com).

Circuit Descriptions Of The Upstream Oxygen Sensors

P0135 Trouble Code Diagnostic Tests (1990-1997 2.3L Ford)

The upstream oxygen sensor on your 2.3L equipped Ford goes by several different names, among them:

  1. O2 sensor before the catalytic converter.
  2. Front oxygen (O2) sensor.
  3. Bank 1 Sensor 1.
  4. O2S11.
  5. HO2S (Heated Oxygen Sensor).
  6. Pre-catalytic converter oxygen (O2) sensor.

It doesn't matter what the upstream 02 sensor is called... the fact of the matter is that it has 4 wires coming out of it and it can be tested very accurately with just a multimeter.

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 P0135 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.

O2 Bank 1 Sensor 1 Circuits
Pin Description
1 Heater Element Ground
2 Heater Element 12 Volts
3 HO2S High Signal
4 HO2S Low Signal

TEST 1: Verifying The Heater Element Is Getting Power

P0135 Trouble Code Diagnostic Tests (1990-1997 2.3L Ford)

The first thing will do, to test the upstream O2 sensor's heater on your 2.3L Ford, is to verify that terminal number 2 of the O2 harness connector is supplying battery power (to the O2 sensor's heater element).

This is a very simple multimeter voltage test (see the illustration in the image viewer).

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 upstream 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.

  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 10 to 12 Volts DC- So far so good since this test result confirms that the upstream oxygen sensor's heater element is getting power.

The next step is to make check that terminal number 1, of the O2 sensor 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 multimeter DID NOT record the indicated voltage- 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 upstream oxygen (O2) sensor itself, on your 2.3L equipped Ford Ranger (Mazda B2300) 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.