How To Test The HO2S-11 Oxygen Sensor's Heater -P0135 (2001, 2002, 2003, 2004 3.0L Ford Escape And 3.0L Mazda Tribute)

As you're probably already aware, a trouble code P0135 points to a problem with the internal heater of the bank 1 sensor 1 oxygen sensor (HO2S-11).

In this tutorial I'm going to explain how to test the HO2S-11 oxygen sensor's heater on the 2001-2004 3.0L Ford Escape (3.0L Mazda Tribute). You'll be able to find out if it's bad or not.

All of the test steps are explained in a step-by-step manner so that you can get to the bottom of the P0135 trouble code lighting up the check engine light on your 3.0L Ford Escape (3.0L Mazda Tribute).

APPLIES TO: This P0135 diagnostic test tutorial applies to the following vehicles:

  1. 3.0L Ford Escape: 2001, 2002, 2003, 2004.
  2. 3.0L Mazda Tribute: 2001, 2002, 2003, 2004.

What Does Trouble Code P0135 Mean?

A P0135: Heated Oxygen Sensor Heater Circuit Malfunction (Bank 1 Sensor 1) trouble code means that there's a problem with the internal heater of the front oxygen sensor for bank 1.

Although the fuel injection computer doesn't know the exact cause of the oxygen sensor's heater problem, it usually boils down to one of the following:

  1. The heater itself has failed.
  2. The O2 sensor's heater is not getting 12 Volts because its power wire has an open-circuit problem.
  3. The O2 sensor's heater is not getting Ground because its Ground wire has an open-circuit problem.
  4. The wires that supply 12 Volts and Ground to the O2 sensor's heater have shorted together.

Now, I'm sure you're wondering where oxygen sensor 1 of bank 1 (HO2S-11) is located, it's on the exhaust manifold that faces your Ford Escape's firewall. You can see its location on the exhaust manifold here: Location Of Oxygen Sensor HO2S-11.

Circuit Descriptions Of The HO2S-11 Oxygen Sensor

As you're probably already aware, the HO2S-11 oxygen sensor has 4 wires coming out of its connector.

Two of those four wires connect directly to the oxygen sensor's heater.

To be a bit more specific, one wire supplies the heater with 12 Volts DC and other wire provides it with Ground.

This Ground comes from the fuel injection computer, of your 3.0L Ford Escape (3.0L Mazda Tribute) and is only supplied when the key is in the ON position.

Here's a brief description of the four wires of the oxygen sensor HO2S-11 connector:

O2 Sensor Bank 1 Sensor 1 (2001-2004 3.0L Ford Escape And Mazda Tribute)
Pin Wire Color Description
1 LT BLU/ORG Heater 12 Volts
2 RED/WHT Heater Ground
3 ORG O2 Signal Ground
4 GRY/LT BLU O2 Signal

NOTE: The bank 1 sensor 1 oxygen sensor's engine wiring harness connector has female terminals (see photo 1 of 2 above). The connector on the oxygen sensor itself has male spade terminals (see photo 2 of 2).

You might be asking yourself: Why does the oxygen sensor need a heater? It's because the O2 sensor needs to reach a certain temperature before it activates.

This operating temperature is 600° F and it has to stay at this temperature to be able to sense the oxygen content of the exhaust stream it's exposed to.

If the O2 sensor doesn't stay hot enough, it will stop sensing the oxygen content of the exhaust.

So the only way to be able to maintain the O2 sensor at a constant 600°+ Fahrenheit, is with an internal heater.

The cool thing is is that we can easily check to see if the O2 sensor is getting 12 Volts and Ground. It's also very easy to test the internal heater by doing a simple multimeter resistance test. In this tutorial I'll show you how.

TEST 1: Making Sure The O2 Sensor's Heater Is Getting 12 Volts

How To Test The HO2S-11 Oxygen Sensor's Heater -P0135 (2001, 2002, 2003, 2004 3.0L Ford Escape And 3.0L Mazda Tribute)

The very first thing that we're going to do is to make sure that the O2 sensor's heater is getting 10 to 12 Volts DC.

These 12 Volts come from the fuse #5 of the Battery Junction Box (underhood fuse/relay box). Fuse #5 is a 15 Amp fuse.

It's the light blue with orange stripe (LT BLU/ORG), of the O2 sensor's 4 wire connector, that carries these 12 Volts from fuse #5 to the heater element of HO2S-11.

The light blue with orange stripe (LT BLU/ORG) connects to the female terminal labeled with the number 1 in the image above.

We're going to do a simple multimeter voltage test to make sure that these 10 to 12 Volts DC are present in this wire.

CAUTION: The oxygen sensor gets and stays very hot even after the engine is off! Perform this test with a completely cold engine. Be careful and take all necessary safety precautions! If you raise your vehicle with a jack, place it on jack stands!

IMPORTANT: To check the presence of 12 Volts, you'll test the oxygen sensor's engine wiring harness connector. This connector has female terminals as shown in the image above.

OK, this is what you'll need to do:

  1. 1

    Locate the HO2S-11 oxygen sensor and disconnect it from its engine wiring harness connector.

  2. 2

    Set your multimeter to Volts DC mode.

  3. 3

    Turn the key ON but don't crank or start the engine (this will power up the O2 sensor's engine wiring harness connector).

  4. 4

    Probe the light blue with orange stripe (LT BLU/ORG) wire of O2 sensor's connector with your multimeter's red test lead

    The LT BLU/ORG wire connects to the female terminal labeled with the number 1 (see photo above).

  5. 5

    With the Key On Engine Off (KOEO), your multimeter should register 10 to 12 Volts DC.

Let's take a look at your test results:

CASE 1: Your multimeter registered 10 to 12 Volts DC. This is the correct and expected test result.

The next step is to verify that the O2 sensor's heater is getting Ground. For this test, go to: TEST 2: Making Sure The O2 Sensor's Heater Is Getting Ground.

CASE 2: Your multimeter DID NOT register 10 to 12 Volts DC. The most likely cause of these missing 12 Volts is a blown fuse or a short (or open) in the wiring.

You'll need to check fuse #5 in the underhood fuse/relay box and make sure it's not blown. If it's blown, replace it and repeat the test.

If the fuse is OK, then your next step is to check the continuity of the LT BLU/ORg wire between the fuse box and the O2 sensor connector.

Ford Vehicles:

  • Escape 3.0L
    • 2001, 2002, 2003, 2004

Mazda Vehicles:

  • Sable 3.0L
    • 2001, 2002, 2003, 2004