Computer-controlled cars require a lot of input to meter air and fuel in the appropriate ratio. The Mustang's O2 (oxygen) sensor is basically a temperature probe; a fuel-rich mixture will make exhaust gases cooler, and a fuel-lean mixture will heat them up. If your O2 sensor becomes unresponsive or fails outright, then fuel economy, horsepower and engine performance will all suffer. Mustangs produced before 1996 use one oxygen sensor before the catalytic converter; those produced later utilize a second one after the converter.
Slip a jack under your Mustang's front sub-frame or engine cross-member. Lift the car about 8 inches and install jack stands under the cross-member then lower the jack.
Locate the O2 sensors. A Mustang can have anywhere from one to four, depending on the specific application. Pre-1996 four cylinders as well as V6 and V8 engines utilizing a single exhaust will have one in the head pipe or the 2-into1 exhaust crossover just below the manifold(s). Older, dual-exhaust cars will often have one in each pipe.
Disconnect the wiring harness from the O2 sensor by pushing down on the connector's locking tab and unplugging it from the sensor.
Slip either a 7/8-inch wrench or O2 sensor removal socket over the O2 sensor and turn it counterclockwise to remove it. An open-ended wrench will usually work for the second O2 sensor, but you may need to use a combination of an O2 sensor socket, universal joint and extension piece to reach the forward-most sensors.
Apply a dab of anti-seize compound onto the threads of the new O2 sensor and screw it in place. Tighten the O2 sensor as much as you can--stripping an O2 sensor bung with hand tools is almost impossible so don't be too concerned about over-torquing it.
Reconnect the wiring harness plug and make sure that the locking tab on the plug fully engages the O2 sensor harness.
Use the floor jack to raise the car slightly off of the jack stands. Remove the jack stands and lower the car.
