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.
Park the Ford on a flat, level surface.
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.
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.
Set the voltmeter to the millivolt scale. Let the engine run until it reaches operating temperature.
Watch the needle on the voltmeter; it should stay fairly consistent at approximately 150 millivolts.
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.
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.
