TEST 2: Making Sure That Ground Is Present
Now that you have confirmed in test one that the O2 sensor is getting power on the LT BLU/ORG wire, we're now going to make sure that the red with white stripe (RED/WHT) wire is feeding the heater element with Ground.
This Ground is provided by your Ford Ranger's fuel injection computer. For this reason, you have to be very careful not to short this wire to battery voltage, or you're going to fry the computer.
The multimeter voltage test, that I am suggesting in the test steps below, is a very safe way to test for this Ground.
NOTE: This test is done on the engine wiring harness connector. This connector has round female terminals.
Let's get testing:
Locate the upstream oxygen sensor and disconnect it from its harness connector.
Locate the RED/WHT wire of the engine wiring harness oxygen sensor connector.
With your multimeter in Volts DC mode, probe the RED/WHT with the black multimeter test lead.
Ground the red multimeter test lead directly on the battery's positive (+) terminal.
Your multimeter should register 10 to 12 Volts DC if Ground is present.
Let's take a look at your test results:
CASE 1: The RED/WHT wire is feeding Ground to the heater. This is the correct test result.
The next step, and the last, is to test the heater element's resistance with a multimeter. For this test go to: TEST 3: Testing The Resistance Of The Heater Element.
CASE 2: The RED/WHT wire IS NOT feeding Ground to the heater. The O2 sensor does not get Ground, it's not going to activate.
The common cause of this missing Ground is an open in the RED/WHT wire between the connector and the fuel injection computers connector.
TEST 3: Testing The Resistance Of The Heater Element
In this last test section, we're gonna' test the front O2 sensor heater element's resistance with a multimeter.
The HO2S 11 heater element's resistance should be between 3 to 30 ohms.
NOTE: This test is done on the connector of the O2 sensor itself. This connector has round male terminals (see photo above).
These are the test steps:
Locate the O2 sensor terminals 1 and 2 of the O2 sensor connector itself (not the engine wiring harness O2 connector).
With your multimeter in Ohms mode, probe terminals 1 and 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 examine your test results:
CASE 1: The heater element's resistance is within specification. This is the correct test result. You can conclude that the O2 sensor's heater is OK (not defective).
CASE 2: The heater element's resistance IS NOT within specification. This test result let you know that the heater element is defective. 3 place the front O2 sensor with a new one.