Where is the crankshaft sensor located ?

The crankshaft position sensor (POS) is located on the oil pan facing the gear teeth (cogs) of the signal plate. It detects the fluctuation of the engine revolution.
After replacement, ECM reprogramming is required.
This vehicle may be subject to voluntary recall, CAMPAIGN I.D. # R0606, that involves the crankshaft position sensor:

Quote: Nissan has determined that on some 2003 - 2004 Altima and Sentra vehicles with the 2.5 liter engine the operation of the crank angle sensor is significantly affected by variations in temperature that occur due to the unique location of the sensor in the engine. Under certain driving conditions, these variations in temperature can be large enough to cause a brief interruption in the signal output from the sensor. If the interruption in the signal from the sensor is so brief that the Electronic Control Module (ECM) logic does not have time to diagnose the condition, the engine may stop running without warning while the vehicle is driven at low speed.

Do you mean Camshaft Position Sensor? On the Maxima's 1993-1999 it's located at the timing Belt Cover. May be yours it's located at the same position.