Open the hood of your Nissan Altima to access the engine bay.
Locate the engine block. It's usually the largest metal block in the engine bay.
Follow the serpentine belt to where it connects to the crankshaft pulley.
Near the crankshaft pulley, look for a small cylindrical sensor with wires connected to it. This is likely the Crankshaft Position Sensor.
In some Nissan Altima models, the CKP sensor may be located behind the starter motor or near the oil filter.
If you're having difficulty finding the CKP sensor, refer to your vehicle's repair manual or consult with a qualified mechanic. The exact location of the sensor may vary depending on the year, model, and engine type of your Nissan Altima.