TEST 2: Verifying The Heater Element Is Getting Ground

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

The second part of the upstream oxygen sensor's heater element test, on your 2.3L Ford, is to make sure that the heater element is getting ground.

Terminal number 1 of the upstream oxygen sensor's harness connector is the one that feeds ground to the heater element.

To get this show on the road, this is what you'll need to do:

  1. 1

    Locate the wire that connects to terminal number 1 of the O2 sensor's 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:

    1. Connect the red multimeter test lead to battery (+).
    2. Connect the black lead to terminal number 1 of the O2 sensor's harness connector.
  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 test result lets you know that your 2.3L Ford Ranger's upstream oxygen sensor's heater element is getting ground.

So far you've confirmed that the upstream O2 sensor's heater element is getting both 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 wire that connects to terminal number 1 of the O2 sensor's harness connector.

By an ‘open’, I mean that the wire is cut somewhere between it and its ground point. To confirm this, you can use a jumper wire to ground this circuit and repeat the test.

Repairing this ground issue will solve the P0135 issue you're having with your 2.3L equipped Ford Ranger (Mazda B2300) pickup.

TEST 3: Testing The Heater Element's Resistance

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

Since you have checked and confirmed that the upstream oxygen sensor on your Ford is getting both juice and ground, the next (and last) step is to check the heater's internal resistance.

This is another multimeter test but with it in Ohms () mode.

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 1 and number 2 of the O2 sensor connector itself (not the engine wiring harness O2 connector).

  2. 2

    With your multimeter in Ohms mode... probe terminals number 1 and number 2 of the O2 sensor itself.

  3. 3

    If all is OK, you should see about 3 to 30 Ω’s 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 your 2.3L Ford Ranger's 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).