TEST 1: Making Sure O2 Sensor Heater Is Getting Power
The first test is to make sure that the right front oxygen sensor is getting power. This power is in the form of 12 Volts and is supplied by the RED wire of the O2 sensor engine wiring harness connector.
This'll be a very simple multimeter voltage test done with the multimeter in Volts DC mode.
By the way, if you don't know which oxygen sensor is which, take a look at the following section: Oxygen Sensor Locations.
IMPORTANT: Perform this (and all of the other tests) with a completely cold engine. The oxygen sensor and the exhaust pipe get extremely hot and stay hot for a long time (after engine shut down). To avoid getting burned, let the engine cool down completely before attempting any of the tests described in this tutorial.
Alright, these are the test steps:
- 1
Unplug the right 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 right 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 RED/WHT 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 P0135 diagnostic trouble code that's lighting up the check engine light.
TEST 2: Making Sure O2 Sensor Heater Is Getting Ground
So far, you've checked and confirmed that the RED wire has power with the key in the ON position (TEST 1).
Your next test step is to check and confirm that the PCM is supplying the heater element with a good Ground.
The wire that feeds the right front O2 sensor (HO2S-11) heater with Ground is the RED/WHT wire.
Alright, these are the test steps:
- 1
Unplug the right front O2 (HO2S11) sensor from the engine wiring harness connector and place your multimeter in Volts DC mode.
- 2
With the black multimeter test lead, check the terminal that connects to the RED/WHT 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 RED/WHT wire is supplying the right 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. So far so good, since this is the correct test result and lets you know that the front right O2 sensor's heater element is getting Ground.
Your next step, and the last one, is to check the O2 sensor's heater element's internal resistance. 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 you've got a problem since this test result tells you that the right front oxygen sensor (HO2S11) heater element IS NOT getting the Ground it needs.
Although it's beyond the scope of this tutorial, you'll need to see if there's an open in the RED/WHT wire between the right front O2 sensor and the PCM. Solving this Ground problem will restore the O2 heater's circuit performance and/or solve the P0135 diagnostic trouble code.
