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 connector on the oxygen sensor itself. To check for ground, you need to test the YEL/LT BLU wire of the engine wiring harness sensor connector.
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:
Locate the YEL/LT BLU 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.
Place your multimeter in Volts DC mode and connect the red multimeter test lead to battery (+).
Now, with the black multimeter test lead, probe the YEL/LT BLU wire of the O2 sensor's harness connector.
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 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 (E150 or E250).
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 P0151 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 P0151: 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:
Locate the O2 sensor terminals number 1 and number 2 of the O2 sensor connector itself (not the engine wiring harness O2 connector).
With your multimeter in Ohms mode probe terminals number 1 and number 2 of the O2 sensor itself.
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 HO2S 21's heater is OK.
CASE 2: Your multimeter showed an open circuit (OL). This confirms that the upstream (Bank 2 Sensor 1) O2 sensor's heater element is fried. Replacing the upstream O2 sensor with a new one will solve the P0151 trouble code lighting up the check engine light (CEL).
Here are some more specifics: Since 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.
Taking all of the above into account you can correctly conclude that the upstream O2 sensor needs to be replaced with a new one.