The front HO2S (heated oxygen sensor) on your 1995 Altima is located in the exhaust manifold, just before the catalytic converter. It is usually screwed into the exhaust manifold with a 22mm wrench.
To replace the front HO2S, you will need to:
1. Disconnect the electrical connector from the sensor.
2. Unscrew the sensor from the exhaust manifold with a 22mm wrench.
3. Apply a small amount of anti-seize lubricant to the threads of the new sensor.
4. Screw the new sensor into the exhaust manifold.
5. Tighten the sensor to the specified torque (18-22 ft-lb).
6. Reconnect the electrical connector to the sensor.
To access this location, you will likely need a car jack and jack stands to provide you with enough space.
