A camshaft position sensor (CMP) is an electronic device that monitors the position and speed of the camshaft. It works with the crankshaft sensor to define the position of the crankshaft drive. The CMP uses a magnet to produce an electronic signal that tells the valves when to open and shut. The CMP then feeds this data to the vehicle's engine control module (ECM). The ECM needs this data to control how much fuel enters the combustion chamber and ignition timing.