Over time your Ford Flex Oxygen Sensor can become fouled or simply fail all together. When this occurs your Ford Powertrain Control Module won’t be able to properly adjust your engine fuel trims. Because of this your engine may run rich (too much fuel) or not enough fuel, which can cause a lot of issues.
Today I will be showing you how to replace a Ford Flex Oxygen Sensor in a 2012 SUV with the 3.5 liter Ford engine in it. Our test truck is a two wheel drive SEL model, although the procedure is the same for the 4WD version as well. The Flex Oxygen Sensor is also known as an 02 sensor or air fuel sensor.
We’ll be replacing the front 02 sensor, as that’s the one that is the EFI sensor. There are two primary or upstream 02 sensors in your Ford. These sensors are the ones that your engine computer relies on to deliver the right amount of fuel, and adjust for maximum efficiency.
When this upstream 02 sensor goes bad, your check engine light will be the first sign of trouble. You can use an OBDII scan tool to check your stored DTC trouble code.
The OBDII scan tool port is on the driver side of your dashboard, located just under the steering column.
Once you’ve pulled your code, you will know which Flex Oxygen Sensor needs replacing. Ours happens to be Bank 1, which is the front upstream 02 sensor facing towards the front of the vehicle.
Symptoms of a failed Flex Oxygen Sensor
- Poor gas mileage
- Lack of power
- Black smoke from tailpipe
- Harsh shifting from transmission
Replacing your upstream Flex Oxygen Sensor
This procedure is best done when your 3.5 liter engine is cold. You will need an 02 socket or an open ended 22mm wrench. Open your Ford Flex hood, and pull back on the front radiator hose to gain access to the 02 sensor.
Once you’ve found the 02 sensor, follow the wiring harness back until you see where it’s mounted to the front valve cover. Pull up to dislodge the upstream 02 sensor from the valve cover itself.
Now depress the locking tab of the plastic upstream 02 sensor harness. Pull apart to disconnect the upstream Flex Oxygen Sensor. Now you have freed the 02 sensor from the rest of your engine harness.
Now pull back on the front radiator hose and carefully unscrew the upstream 02 sensor. Once you have the old unit removed, you can install the new oxygen sensor. Because of the heat shields over the exhaust manifold, it’s best to remove this using an 02 socket. Carefully dab some anti-seize to the thread area, making sure not to get any on the actual air fuel element.
Once you’ve got the front 02 sensor installed, you can use a OBDII scan tool to clear your stored DTC trouble code. You have now replaced your front or primary Flex Oxygen Sensor. Have any questions about this guide? Leave us a comment below and let us know!