TEST 2: Making Sure The O2 Sensor's Heater Is Getting Ground
If you've reached this point, you've made sure that the LT BLU/ORG wire is feeding 10 to 12 Volts to the HO2S-21 sensor's heater.
Our next step is to make sure that the O2 sensor's heater is getting Ground. We'll do this with a simple multimeter voltage test.
The wire that provides this Ground, to the heater element, is the red with white stripe (YEL/LT BLU) wire. In the image above, the terminal labeled with the number 2 is the one that connects to the YEL/LT BLU wire.
If Ground is present, then we'll move on to the last test, which is testing the resistance of the heater element within the O2 sensor.
NOTE: To check the presence of Ground, you'll test the oxygen sensor's engine wiring harness connector. This connector has female terminals as shown in the image above.
IMPORTANT: Ground is provided by the fuel injection computer of your Ford Escape (Mazda Tribute). So, be careful and don't short the Ground wire to battery power or you'll fry the fuel injection computer. The multimeter voltage test I'm suggesting below is a safe way of testing for the presence of Ground in this wire.
OK, this is what you'll need to do:
Disconnect the HO2S-21 sensor from its engine wiring harness connector.
Set your multimeter to Volts DC mode and turn the key ON but don't crank or start the engine.
Probe the red with white stripe (YEL/LT BLU) wire of O2 sensor's connector with the black multimeter test lead.
The YEL/LT BLU wire connects to the female terminal labeled with the number 2 (see photo above).
Connect the red multimeter test lead to the battery positive (+) post.
With the Key On Engine Off (KOEO), your multimeter should register 10 to 12 Volts DC.
Let's take a look at your test results:
CASE 1: Your multimeter registered 10 to 12 Volts DC. This is the correct test result and it confirms that the HO2S-21 sensor's heater element is getting Ground.
The next step is to verify that the heater's resistance is within specification. For this test, go to: TEST 3: Testing The Heater Element's Resistance.
CASE 2: Your multimeter DID NOT register 10 to 12 Volts DC. This test result tells you that the HO2S-21 sensor's heater element is not getting Ground.
The most likely cause of this missing Ground is an open-circuit problem in the Ground wire between the O2 sensor's connector and the fuel injection computer.
TEST 3: Testing The Heater Element's Resistance
For our last test, we're going to test the resistance of the heater element itself. This is a very simple test since it simply involves measuring the resistance of the heater element with a multimeter in Ohms mode.
The resistance specification of the heater element is 3 to 30 Ohms at room temperature.
If the resistance is not within the specification, then you can conclude that the O2 sensor is bad and needs to be replaced.
OK, this is what you need to do:
Locate the female 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.
Place your multimeter in Ohms mode.
With your multimeter in Ohms mode probe terminals number 1 and number 2 of the O2 sensor's connector.
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: The oxygen sensor's heater resistance is within specification. This is the correct test result and tells you the oxygen sensor's heater element is OK.
CASE 2: The oxygen sensor's heater resistance IS NOT within specification. This confirms that the HO2S-21 sensor's heater element is fried.
You can conclude that the HO2S-21 sensor is bad and the cause of the P0155 trouble code if you have:
- Confirmed that the HO2S-21 sensor's heater element is getting 10 to 12 Volts (TEST 1).
- Confirmed that the HO2S-21 sensor's heater element is getting Ground (TEST 2).
- In this test you have confirmed that the heater element's resistance is not within the indicated specification.
Taking all of the above into account you can correctly conclude that the HO2S-21 sensor needs to be replaced with a new one.
Location Of Oxygen Sensor HO2S-21
The Bank 2 Sensor 1 oxygen sensor (HO2S-21) is located on the exhaust manifold facing the radiator.
The illustration above is of the exhaust manifold that connects to bank 2 of the engine (bank 2 is the bank for cylinders #2, #4, and #6).
More 3.0L Ford Escape Tutorials
You can find a complete list of tutorials in this index: Ford 3.0L Index Of Articles.
Here's a small sample of the 3.0L Ford Escape tutorials you'll find in the index:
- How To Test The Fuel Pump (2001-2004 3.0L Ford Escape).
- How To Test The Fuel Injectors (2001-2004 3.0L Ford Escape).
- How To Test Engine Compression (3.0L Ford Escape).
- How To Test For A Blown Head Gasket (3.0L Ford Escape).
If this info saved the day, buy me a beer!