Your 2003 Mitsubishi Galant came with either a 2.4-liter four-cylinder or a 3.0-liter six-cylinder engine. Both engines use oxygen sensors to detect how much oxygen is in the exhaust system when it exits the engine and the catalytic converter. These sensors have an impact on your fuel economy and engine performance, because as the engine runs the Galant's onboard computer uses information received from the oxygen sensors to change the air-to-fuel ratio accordingly. A faulty oxygen sensor will likely turn on the check engine light and will cause you to fail an emissions tests.
Park the vehicle on a level surface and set the parking brake. Lift the front of the vehicle into the air with the floor jack and place the jack stands under the front sub frame. Lower the floor jack until the vehicle rests securely on the jack stands. Allow the engine and exhaust to cool until the exhaust is cool to the touch.
Locate the connector for the front oxygen sensor, it is on the front of the engine next to where the radiator hose connects to the engine. Trace the front oxygen sensor pigtail from the sensor to the connector. Squeeze the locking tab and separate the pigtail from the vehicle wiring harness.
Place the oxygen sensor socket over the front oxygen sensor, making sure the pigtail extends through the slot in the socket. Heat the area around the oxygen sensor with the propane torch and rotate the sensor counterclockwise with the ratchet and oxygen sensor socket. Remove the oxygen sensor from the vehicle.
Open the driver side door and pull the carpet back from the firewall and accelerator pedal area to expose the rear oxygen sensor pigtail. Continue to pull back the carpet until the connector is accessible at the base of the center console. Squeeze the locking tab and separate the oxygen sensor pigtail from the vehicle wiring harness. Push the pigtail grommet through the firewall, followed by the connector. Remove the sensor as explained in Step 3.
Clean the front sensor threads, if the sensor is being reused. Apply a coat of copper anti-seize to the sensor threads and thread the sensor into the exhaust pipe. Place the sensor socket over the sensor with the pigtail extending through the slot. Tighten the sensor to 32 foot-pounds with a torque wrench.
Route the pigtail up towards the radiator hose and engine connection. Plug the front oxygen sensor pigtail into the vehicle wiring harness. Pull on the connectors to make sure they are properly connected.
Install the rubber grommet over the rear oxygen sensor pigtail and push the pigtail through the firewall. Push the grommet through the firewall so that it creates a good seal. Connect the oxygen sensor pigtail to the wiring harness at the base of the center console. Repeat step 1 to install the rear oxygen sensor to the rear exhaust pipe.
Lift the vehicle off of the jack stands with the floor jack and remove the jack stands. Lower the vehicle to the ground.
Lift the front of the vehicle into the air with the floor jack and place the jack stands under the front sub frame. Lower the floor jack until the vehicle rests securely on the jack stands. Remove the floor jack.
Locate the front left oxygen sensor connector that is just below radiator hose to thermostat housing connection. Squeeze the locking tab and pull the pigtail from the harness. Locate the front right oxygen sensor connector that is located at the top of the engine next to the intake manifold. Squeeze the locking tab and separate the sensor pigtail from the wiring harness.
Place the oxygen sensor socket over either oxygen sensor making sure the pigtail extends through the slot in the sensor socket. Heat the area around either socket with a propane torch and rotate the sensor counterclockwise with the sensor socket and ratchet. Remove the remaining oxygen sensor, if needed.
Clean the threads of the oxygen sensor with the wire brush and apply a coat of copper anti-seize compound to the threads. Thread the oxygen sensor into the exhaust pipe. Tighten the oxygen sensor to 32 foot-pounds with the oxygen sensor socket and torque wrench. Route the left sensor pigtail to the harness connect below the thermostat housing and route the right pigtail to the wiring harness connector next to the intake manifold. Push the connectors into place and pull on them to ensure a proper connection.
Lift the vehicle off of the jack stands with the floor jack and remove the jack stands. Lower the vehicle to the ground.
Lift the car into the air according to step 1 of “3.0-Liter Front Oxygen Sensor Replacement”. Pull back the carpet from the firewall and center console on the driver side to expose the rear oxygen sensor and wiring harness connectors.
Squeeze the locking tab and slide the oxygen sensor connector from the wiring harness connector. Push the rubber grommet through the firewall to the outside of the vehicle, followed by the pigtail connector. Remove the rear oxygen sensor from the exhaust pipe with the oxygen sensor socket and ratchet.
Clean the oxygen sensor threads with a wire brush and apply a coat of copper anti-seize to the threads. Hand thread the oxygen sensor into the exhaust pipe. Tighten the sensor to 32 foot-pounds with the torque wrench and oxygen sensor socket.
Place the rubber grommet over the sensor pigtail. Push the pigtail through the hole in the firewall into the car. Push the rubber grommet into the hole so that it makes a good seal between the inside and outside of the vehicle.
Connect the oxygen sensor pigtail to the vehicle wiring harness at the base of the center console. Pull on the connectors to make sure they are properly connected. Press the carpet back into place.
Raise the front of the vehicle off of the jack stands with the floor jack. Remove the jack stands from under the vehicle and lower the vehicle to the ground. Remove the floor jack.
