TEST 2: Verifying The Heater Element Is Getting Ground

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

The second test that we'll do is to check that your 2.0L Escape's upstream O2 sensor's heater element is getting Ground.

The upstream O2 sensor's heater element is fed Ground by the red with white stripe (RED/WHT) wire (of the engine wiring harness HO2S 11 electrical connector).

We'll do a very simple multimeter voltage test to see if the RED/WHT wire is feeding the heater with ground or not.

IMPORTANT: The illustration of the connector above is of the connector on the oxygen sensor itself. To check for ground, you need to test the RED/WHT wire of the engine wiring harness sensor connector.

CAUTION: The heater element gets ground from the PCM (internally). Be careful that you don't accidentally or intentionally short this wire to battery power or you'll fry you Ford's PCM. Doing the described multimeter voltage test below is a safe way to test this circuit for ground.

These are the test steps:

  1. 1

    Locate the RED/WHT wire of the O2 sensor's engine wiring harness connector.

    NOTE: Remember, you'll test the wire that's on the engine wiring harness connector side and NOT on the O2 sensor itself.

  2. 2

    Place your multimeter in Volts DC mode and connect the red multimeter test lead to battery (+).

    Probe the RED/WHT wire, of the O2 sensor's harness connector, with the black multimeter test lead.

  3. 3

    With the Key On, engine Off, this wire should have 10 to 12 Volts DC.

Let's take a look at your test results:

CASE 1: The multimeter registered 10 to 12 Volts DC. This is the correct and expected test result and confirms that the HO2S 11's heater is getting Ground.

Now that you have verified the basics (power and Ground) the next step is to check the heater element's resistance with your multimeter. For this test, go to: TEST 3: Testing The Heater Element's Resistance.

CASE 2: The multimeter DID NOT register 10 to 12 Volts DC. Re-check all of your connections and make sure you're testing the correct terminal.

If your multimeter still doesn't register the 10 to 12 Volts DC, then the most likely cause of this missing Ground is an ‘open’ in the RED/WHT wire between the O2 sensor's harness connector and your Ford's PCM.

TEST 3: Testing The Heater Element's Resistance

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

So far, you have confirmed that:

  1. A P0135 trouble code is lighting up the check engine light on your 2.0L Ford Escape's instrument panel.
  2. The LT BLU/ORG wire has power (TEST 1).
  3. The RED/WHT wire has Ground (TEST 2).

For our last test, we're gonna' check the O2 sensor's heater element's resistance. If the resistance is not within specification, then we now know the O2 sensor is bad and the cause of the P0135: P0135: HO2S 11 Circuit Malfunction trouble code.

NOTE: Just a reminder that the upstream oxygen sensor has to be completely cold before proceeding with this test since the manual calls for the O2 sensor to be at room temperature for the resistance test.

OK, this is what you need to do:

  1. 1

    Locate the O2 sensor terminals number 3 and number 4 of the O2 sensor connector itself (not the engine wiring harness O2 connector).

  2. 2

    With your multimeter in Ohms mode, probe terminals number 3 and number 4 of the O2 sensor itself.

  3. 3

    If all is OK, you should see about 3 to 30 Ωs on your multimeter.

    If the heater element is fried, your multimeter will show an open (usually indicated by the letters OL) or a number over 10 K Ωs.

Let's take a look at your test results:

CASE 1: Your multimeter confirmed the indicated resistance. This test result tells you that the pre-catalytic converter oxygen (O2) sensor's heater is OK.

CASE 2: Your multimeter showed an open circuit (OL). This confirms that the upstream (Bank 1 Sensor 1) O2 sensor's heater element is fried. Replacing the upstream O2 sensor with a new one will solve the P0135 trouble code lighting up the check engine light (CEL).

Here are some more specifics: Since you have:

  1. Confirmed that the upstream O2 sensor's heater element is getting power (TEST 1).
  2.      -AND-
  3. Confirmed that the upstream O2 sensor's heater element is getting Ground (TEST 2).
  4.      -AND-
  5. In this test you have confirmed that the heater element's resistance is out of specification.

...You can correctly conclude that the upstream O2 sensor needs to be replaced with a new one.

Ford Vehicles:

  • Escape 2.0L
    • 2001, 2002, 2003, 2004

Mazda Vehicles:

  • Tribute 2.0L
    • 2001, 2002, 2003, 2004