The first thing you should do when a check engine light comes on in your car is to go to an auto parts store to have them hook up a diagnostic tool to your car's computer system. The diagnostic tool will produce an error code to alert you to the exact problem. An error code of P0440 means there is something wrong with the emission evaporative system. A bad oxygen sensor is one of the major reasons the check engine light comes on. Here how to replace a bad front oxygen sensor on a 2002 Subaru Impreza.
Accessing the Front Sensor
The front oxygen sensor on a 2002 Subaru Impreza is located behind the front passenger side fender well. You need to jack up the car and remove the front passenger side tire. Make sure you set the car on jack stands in case the jack fails, and you should put a wood block behind the back passenger wheel so the car doesn't roll backward off of the jack stands.
Once the tire is off, remove the clips holding the plastic cover to the inside of the wheel well. Remove the cover and set it aside. You can pop the clips out with a flathead screwdriver. The oxygen sensor is located right behind the lower part of the cover and the chassis.
The space between the chassis and the engine is very limited, and it is hard to get an O2 sensor wrench onto the sensor. An O2 sensor wrench has a gap in it so you can slip it over the wires of the sensor without breaking them. An alternative to the O2 wrench is a pair of channel locks. You can slide the channel locks up into the space between the chassis and the engine block where the sensor is located.
Slide the channel locks up into the space between the chassis and the engine block, and grab onto the nut on the sensor. Turn the nut on the sensor counterclockwise to unscrew the sensor from the engine block. Locate the electrical harness along the wires going to the sensor, and disconnect the harness. You can now pull the old sensor out of the car.
Install New Oxygen Sensor
Screw the new sensor into the engine block and tighten it with the channel locks. Reconnect the electrical wire harness. Put the plastic wheel well cover on, and remount the tire. Lower the car and remove the wood block at the rear wheel.
Start the car. The check engine light will still be on after you have replaced the oxygen sensor. You need to take the car out for a long drive until the computer cycles through its engine check system. The check engine light will go off after the computer finishes checking everything. You usually need to drive the car for a half hour or more for the computer system to cycle through its rotation of checking the engine for any malfunctions.
If the prospect of changing a sensor sounds too involved, take you car to an engine repair center like American Transmission Center and have this service performed for you.