Oxygen Sensor Heater Test -P0141 (1997-1998 4.2L V6 F150, F250)

TEST 2: Verifying The Heater Element Is Getting Ground

If you've reached this point, you have confirmed that HO2S 21's heater element is being fed power. The next step is to check that it's getting Ground.

The left front oxygen sensor's heater element is fed Ground by the yellow with light blue stripe YEL/LT BLU wire (of the engine wiring harness HO2S 21 electrical connector).

We can do a very simple multimeter voltage test to see if Ground is indeed present or not.

IMPORTANT: The illustration of the connector above is of the engine wiring harness O2 sensor connector. This connector has female terminals.

CAUTION: The heater element gets Ground from your Ford's PCM (internally). Take care not to accidentally or intentionally short this wire to battery power or you'll fry your Ford's PCM. The multimeter voltage test below is a safe way to test this circuit for Ground.

These are the test steps:

1. 1

   Disconnect the O2 sensor from its engine wiring harness connector.

2. 2

   Place your multimeter in Volts DC.

3. 3

   Connect the red multimeter test lead to the battery positive (+) post.

4. 4

   Turn the key to the RUN position but don't crank or start the engine.

5. 5

   Gently probe the female terminal of the connector that connects to the YEL/LT BLU wire with the black multimeter test lead.
   
   NOTE: This test is done on the O2 sensor's 4-wire engine wiring harness connector. This connector has female terminals.

6. 6

   Your multimeter should register 10 to 12 Volts DC.

Let's take a look at your test results:

CASE 1: The multimeter confirms that the YEL/LT BLU wire is feeding Ground since it registered 10 to 12 Volts DC. This test result lets you know that the upstream oxygen sensor's heater element is getting Ground on your 4.2L Ford F150 (F250).

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 confirms that the YEL/LT BLU wire IS NOT feeding Ground since it 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 YEL/LT BLU wire between the O2 sensor's harness connector and your Ford's PCM.

TEST 3: Testing The Heater Element's Resistance

So far, your HO2S 21 test results have revealed that:

• A P0141 trouble code is lighting up the check engine light.
• The HO2S 21's internal heater is being fed 12 Volts by the RED wire (TEST 1).
• The HO2S 21's internal heater is getting Ground by the YEL/LT BLU wire (TEST 2).

Now for the last test, which is to check the O2 sensor's heater element's resistance and see if it's within the factory specification. If the resistance is not within specification, then we now know the O2 sensor is bad and the cause of the P0141: HO2S 21 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

   Disconnect the O2 sensor.

2. 2

   Locate the male terminals labeled with the number 1 and number 2 of the O2 sensor connector itself (not the engine wiring harness O2 connector).
   
   NOTE: This connector has male terminals and belongs to the O2 sensor itself.

3. 3

   Place your multimeter in Ohms mode.

4. 4

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

5. 5

   If all is OK, you should see about 3 to 30 Ohms (Ω) 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 Ω.

Let's take a look at your test results:

CASE 1: Your multimeter confirmed the indicated resistance. This test result tells you that HO2S 21's heater is OK.

CASE 2: Your multimeter showed an open circuit (OL). This confirms that the O2 sensor's heater element is fried.

You can conclude the O2 sensor is bad and needs replacement if you have:

• Confirmed that the left front O2 sensor's heater element is getting power (TEST 1).
• Confirmed that the left front O2 sensor's heater element is getting Ground (TEST 2)
• In this test, you have confirmed that the heater element's resistance is out of specification.

More 4.2L V6 Ford F150 And F250 Diagnostic Tutorials

You can find a complete list of 4.2L Ford F150/F250 tutorials and wiring diagrams in this index:

• 4.2L Ford F150 And F250 Index of Articles.

Here's a small sample of the tutorials you'll find in the index:

• How To Test The Fuel Pump (1997-2003 4.2L V6 Ford F150, F250).
• How To Test The Fuel Injectors (2001-2003 4.2L V6 Ford F150, F250).
• How To Test The Throttle Position Sensor (1997-2000 4.2L V6 Ford F150, F250).
• How To Test The Ford EGR Valve EGR Vacuum Solenoid, DPFE Sensor.
• How To Test The Coil Pack (Ford 3.0L, 3.8L, 4.0L, 4.2L).