2002 nissan sentra gxe 1.8 bank2 sensor1 federal emission.Which sensor do I replace?Top/bottom,driver/pass side?

There is 4 of them, one behind each of the exhaust manifolds. Driver side being bank 1 sensor1, passenger side being bank 2 sensor1. And one behind each of the catalytic converters. Driver side being bank1 sensor2, passenger side being bank2 sensor2.

O2 sensor/s faulty

should be the one with the whit sheathing around the wires that are part of sensor.bank 1 is right side(firewall side) #2 is the sensor after cat converter.im almost positive it is the white and not the red sheathing.

Air/Fuel Sensors are mounted upstream of the catalytic converter. They are located on the exhaust manifold side of the catalytic converter (pre-converter - Sensors 1). Oxygen Sensors are mounted downstream of the catalytic converter (post-converter - Sensors 2). The catalytic converter has two chambers (left and right). Bank1 is left, drivers side. Bank2 is right, passengers side. The Bank2 Sensor1 sensor (Air/Fuel Sensor) is the one located on the 'right and ahead' of the converter. The PowerTrain Control Module (PCM) is reporting that the heater or circuit ( wiring / plug-in) of the sensor is malfunctioning.

There are two speed sensors one is called a revolution sensor that is located on the case of the transmission. You can take the driver side wheel off and see the sensor on the top of the case of the transmission. A 10mm bolt is holding it in. Easy fix

Engine coolant temp sensor should be on the drivers side of the engine block. Follow your top radiator hose from the radiator to the engine block. near that you should see a sensor (it has a clip on it where the wires attach).

Here is a diagram of the 1.8 exhaust from the service manual, you can obtain a full factory service manual for relatively cheap from Promanuals.net here.

Bank 1 is the same side as cylinder #1 and bank 2 is the same side as cylinder #2. Good luck and hope this helps.

you need a haynes manual . because crank sensor can be behind crank pulley or on bottom side of engine that why you need a manual save you time.

buy a used unit from the junk yard.