I have a Nissan altima 2003 it will start but if you turn it of it will not start, after cool it will start.

SOURCE: 2003 Nissan Altima?

Where is the sensor placed on the car?

SOURCE: 2003 Nissan Altima heat problems

take to dealership , that also is a recall problem with alot of altimas , they need to flush system and check water valve , got mine fixed working great ever since , never hesitate to ask dealership about recalls , they have to fix for free

SOURCE: crankshaft and camshaft sensors location in nissan altima 2003

You are incorrect sir. There is a very good Youtube video showing you exactly where it is that is how I changed mine. The Crankshaft is located behind the engine block below and around from the cam sensor which is easly spotted looking at the engine. The crankshaft is in a difficult spot but once you figure out where it is it's easy...ish hope this helps

SOURCE: 2003 Nissan Altima, Air condition makes loud noise when the unit is turn on. It slows down where u cant almost hear it after you turn the fan off.

i had same problem turned out it was the bearings in ac compressor pulley.cut ac belt off. get new bearings installd in pulley.

SOURCE: 2003 Nissan Altima Heater Not Working

check ur water level