TEST 1: Making Sure O2 Sensor Heater Is Getting Power
To get our P0141: Heated Oxygen Sensor (HO2S-21) Circuit Malfunction under way, we're gonna make sure that the left front oxygen sensor (HO2S21) is getting power.
This power is provided by the RED wire of the oxygen sensor's engine wiring harness connector and is in the form of battery power (10 to 12 Volts DC).
Well check/confirm this voltage with our trusty multimeter.
Alright, these are the test steps:
- 1
Unplug the left front O2 sensor from the engine wiring harness connector and place your multimeter in Volts DC mode.
- 2
With the red multimeter test lead, check the terminal that connects to the RED wire of the oxygen sensor wiring harness connector.
IMPORTANT: Be careful and don't damage the terminal in the connector with the multimeter test lead. - 3
Ground the black multimeter test lead on a clean and rust free spot on the chassis or engine. Better yet, my suggestion to you would be to connect the lead directly to the battery negative (-) terminal.
- 4
Have a helper turn the key on, but NOT start the engine. This will power up the PCM power relay and get juice flowing down the RED wire to the O2 sensor you're currently testing.
- 5
Your multimeter should register 12 Volts DC if the RED wire is supplying the left front O2 sensor's heater with juice.
Let's take a look at what your test results mean:
CASE 1: Your multimeter registered 10 to 12 Volts. So far so good, since this is the correct test result.
Your next step is to make sure the YEL/LT BLU wire is feeding the O2 heater with Ground. Go to: TEST 2: Making Sure O2 Sensor Heater Is Getting Ground.
CASE 2: Your multimeter DID NOT register 10 to 12 Volts. Double check your multimeter connections and retest.
If your multimeter still does not register 12 Volts, then you've got a problem. This lack of 12 Volts is what's causing the trouble code to light up the check engine light (CEL) on your Ford.
Solving this 12 volt power problem will restore the O2 heater's circuit performance and/or solve the P0141 diagnostic trouble code that's lighting up the check engine light.
TEST 2: Making Sure O2 Sensor Heater Is Getting Ground
The next test, in our P0141 trouble code diagnostic, is to make sure that the left front oxygen sensor's heater is getting Ground.
This Ground is provided by the PCM. Yes, this Ground wire DOES NOT connect directly to any point on your Ford's engine or the battery negative (-) terminal.
The wire that feeds the left front O2 sensor (HO2S-21) heater with Ground is the YEL/LT BLU wire.
Alright, these are the test steps:
- 1
Unplug the left front O2 (HO2S21) sensor from the engine wiring harness connector and place your multimeter in Volts DC mode.
- 2
With the black multimeter test lead, probe the terminal that connects to the YEL/LT BLU wire of the oxygen sensor wiring harness connector.
IMPORTANT: Be careful and don't damage the terminal in the connector with the multimeter test lead. - 3
Connect the red multimeter test lead to the battery positive (+) terminal.
- 4
Have a helper turn the key on, but NOT start the engine. This will power up the PCM, since it's the PCM that feeds this Ground to the O2 sensor.
- 5
Your multimeter should register 12 Volts DC if the YEL/LT BLU wire is supplying the left front O2 sensor's heater with Ground.
Let's take a look at what your test results mean:
CASE 1: Your multimeter registered 10 to 12 Volts. This is the correct test result and confirms that the left front oxygen sensor's heater element is getting Ground.
The last thing we'll do, before confirming the O2 sensor's heater is OK (or not) is to check its internal resistance. For this test go to: TEST 3: Checking The O2 Heater's Resistance.
CASE 2: Your multimeter DID NOT register 10 to 12 Volts. Double check your multimeter connections and retest.
If your multimeter still DOES NOT register 12 Volts, then the left front oxygen sensor (HO2S21) heater element IS NOT getting the Ground it needs and this is setting the P0141 trouble code lighting up the check engine on your Ford F150 (F250).
Although it's beyond the scope of this tutorial, you'll need to see if there's an open in the YEL/LT BLU wire between the left front O2 sensor and the PCM. Solving this Ground problem will restore the O2 heater's circuit performance and/or solve the P0141 diagnostic trouble code.
