Ford O2 sensors, more commonly referred to as oxygen sensors, are
designed to read the exhaust emissions from your engine prior to
reaching the catalytic convertor. Often times oxygen sensor codes are
generated when the sensor detects another issue, but the sensors
themselves can malfunction. If you suspect your Ford oxygen sensor has
failed, you can perform a fairly simple test to determine conclusively
if you should replace the sensor or look elsewhere for the source of the
malfunction.

    1. Step 1
Park the Ford on a flat, level surface.

    2. Step 2
Consult the vehicle specific service manual to determine which wire
from the sensor is the sensing wire that provides information to the
power train control module.

    3. Step 3
Disconnect the oxygen sensor from the engine wiring harness and plug
the tip of the voltmeter¡¯s probe into the terminal for the sensing
wire.  Depending on what year, make and model of Ford you are working
on, you  may need to jack the front end up first and support it on jack
stands to  give you the proper clearance to reach the oxygen sensor.

    4. Step 4
Set the voltmeter to the millivolt scale. Let the engine run until it
reaches operating temperature.

    5. Step 5
Watch the needle on the voltmeter; it should stay fairly consistent  at
approximately 150 millivolts.

    6. Step 6
Have your helper tap the throttle to raise the engine RPMs while you
continue watching the needle. The needle should plummet when the engine
RPMs increase. Once the RPMs begin to fall, the needle should rise again
and then settle back where it was in step 5 once the engine returns to
idle.

    7. Step 7
Reconnect the oxygen sensor to the wiring harness if you have
determined it is in proper working order. If not, allow the engine to
cool back down and replace the oxygen sensor with a new one. Make sure
your replacement sensor is identical to the one you are replacing in all
aspects.More information can be found online at
http://www.cars-repair.com/ <http://www.cars-repair.com/>