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O2 Sensor Replacement

How to Replace O2 Sensors on a Ford F-150







O2 sensors measure oxygen levels in the exhaust stream. The engine's power control module uses this information to adjust and maintain the proper air-to-fuel ratio. Faulty or warn sensors send poor or irrational signals to the PCM, resulting in a less than ideal air-to-fuel ratio. This is associated with poor performance, throttle response, and fuel economy. Replacing the O2 sensors is a time consuming task that requires a variety of tools, but it's perfectly acceptable to only replace the sensors that are triggering a check engine light (in most cases there is a different trouble code for each individual sensor, so isolating the faulty sensor is as easy as pulling the trouble codes with a scan tool). If you have the time (and money), replace them all for peace of mind. Afterall, if one fails, the others can't be far behind. There is a difference between upstream and downstream sensors, so make sure you purchase the right ones. A truck has two heated (upstream) and two unheated (downstream) sensors.


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O2 sensor locations on a Ford F-150

• Map of the O2 sensor locations (left image). O2 sensors are located both upstream and downstream of the catalytic converters.
• The O2 sensors are 22 mm, but you can get away with using a 7/8" if using a flare nut wrench. Do not attempt to use an open end 7/8" wrench, as you will likely round the corners of the O2 sensor nut.

O2 sensor removal tools

• Before beginning, disconnect the negative battery cable. This will reset the PCM and allow it to calibrate for the new sensor readings.
• Do to the lack of space around the sensors, a variety of 22mm (or 7/8") tools will be used.
• A flare nut wrench and O2 sensor removal tool are the tools of choice since they eliminate the chance of stripping the O2 sensors, which are very tight.

Drivers side O2 sensor

• Driver side front sensor - The front sensor on the driver's side is fairly simple to remove with a 22mm wrench. You will need to crawl under the vehicle to access it. A flare nut wrench is recommended to ensure you don't round the nut trying to free it.

Drivers side rear O2 sensor

• Driver side rear sensor - Since this truck is 4 wheel drive, the transfer case limits access to the rear of the catalytic converter, which is where the O2 sensor is located. Loosening the exhaust clamp located behind the catalytic converter and unbolting the ball joint that connects the exhaust system to the exhaust manifold allowed us to lower a 2-3 foot section of the exhaust system that contains the 2 catalytic converters. This gave us just enough room to access the sensor, remove it with a 22mm flare nut wrench, install the new sensor, and slide the exhaust back into place. This may not be required for all applications, but it made the task much easier in this case.

Passenger side front O2 sensor

• Passenger side front sensor - The passenger side front O2 sensor is located directly behind the ball joint where the exhaust manifold connects to the exaust system. This truck has the sensor pointing upwards, but some trucks will have the sensor pointing inward. Removal of the inner fender (plastic liner inside the front fender) as well as the transmission dipstick tube may be required. We were able to remove this sensor by unfastening half of the inner fender and pulling it out of the way. This is generally the most time consuming sensor to remove due to its space restricted location, so be prepared to use all the tools in your arsenal.

Passenger side rear O2 sensor

• Passenger side rear sensor - Accessing this sensor is similar to accessing the driver side rear, but without having to disconnect the exhaust system.

• If you damage the threads on an O2 sensor bung - If you happen to strip the threads from an O2 sensor bung, most auto parts stores carry a specific tool to repair them. The threads are M18, and if you need the tool ask for a "M18 spark plug/O2 sensor thread repair tool". A normal thread tap or chaser is difficult to use (or in most cases simply won't fit); the dedicated tool is much shorter and will work in the limited workspace. Chase the threads by applying a liberal amount of cutting fluid (or even motor oil) to the cutting threads and carefully drive the tool into the sensor bung, being sure to clean the cutting edge frequently by backing it out a full turn for every 1 to 2 rotations (as you would when normally tapping a thread). Once the threads have been repaired, clean the area thoroughly with a solvent (brake or carb cleaner) to remove the oil, then install the sensor.