Oxygen Sensor Heater Test -P0135 (2001-2004 2.0L Ford Escape -2.0L Mazda Tribute)

Your 2.0L Ford Escape (or 2.0L Mazda Tribute) comes equipped with 2 oxygen sensors. When the upstream oxygen sensor's internal heater fails, you're gonna' see a diagnostic trouble code P0135: HO2S 11 Circuit Malfunction lighting up the check engine light on your instrument panel.

The cool thing is that testing the upstream oxygen sensor's heater is pretty easy and in this tutorial I'll help you test it.

In case you're wondering what tools you'll need: All you'll need is a multimeter and in 3 basic tests you can find out if it's fried or not.

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

  1. Pre-Catalytic Converter Oxygen (O2) Sensor.
  2. HO2S 11 (Heated Oxygen Sensor Bank 1 Sensor 1).
  3. Upstream Oxygen (O2) Sensor.
  4. Oxygen Sensor Bank 1 Sensor 1.

Here are the contents of this tutorial:

  1. Circuit Descriptions Of The Upstream Oxygen Sensor.
  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 2.0L Ford Diagnostic Tutorials.

En Español You can find this tutorial in Spanish here: Calentador del Sensor de Oxígeno -P0135 (2001-2004 2.0L Ford Escape) (at: autotecnico-online.com).

Circuit Descriptions Of The Upstream Oxygen Sensor

Oxygen Sensor Heater Test -P0135 (2001-2004 2.0L Ford Escape -2.0L Mazda Tribute)

The upstream oxygen sensor (HO2S 11), on your 4 cylinder 2.0L Ford Escape (2.0L Mazda Tribute) is a 4 wire oxygen sensor.

2 wires are for the section, of the front O2 sensor, that deals with sensing the oxygen content of the engine's exhaust. The other 2 wires are the ones that feed power and ground to the heater element.

Below, you'll find the color of the wires of the engine wiring harness oxygen sensor connector for sensor HO2S 11:

Upstream Oxygen Sensor Pinout
(2001-2004 2.0L Ford Escape)
Pin Wire Color Description
1 RED/WHT Heater Ground (-)
2 LT BLU/ORG Heater Power (+)
3 ORG O2 Signal Ground
4 GRY/LT BLU O2 Signal

TEST 1: Verifying The Heater Element Is Getting Power

Oxygen Sensor Heater Test -P0135 (2001-2004 2.0L Ford Escape -2.0L Mazda Tribute)

Your 2.0L Ford Escape's upstream O2 sensor's heater element gets power from the light blue with orange stripe (LT BLU/ORG) wire of its engine wiring harness connector.

So, to get our P0135 diagnostic under way... the first thing we're gonna' do is check that this wire does have 10 to 12 Volts DC with the key on engine off (KOEO).

CAUTION: Perform this test with a completely cold engine to avoid getting burned from a hot O2 sensor or exhaust manifold! Be careful and take all necessary safety precautions!

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

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 LT BLU/ORG wire of the engine wiring harness oxygen sensor connector.

  3. 3

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

    Ground the black multimeter 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: Your multimeter confirms that the LT BLU/ORG wire has 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 the RED/WHT wire, of the O2 sensor 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 multimeter confirms that the LT BLU/ORG wire DOES NOT have 10 to 12 Volts DC- 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 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.