(CKP) Crankshaft Position Sensors
(EFI) Electronic fuel injection engines use the crankshaft position sensor signal for fuel and injector timing. It's often used along with the camshaft position sensor to compute ignition timing. Distributorless ignition systems require a crank sensor. They're typically diagnosed with a scan tool and an ohmmeter.
A loose plug on a crankshaft position sensor will result in an intermittent circuit, stalling, and a no-start condition. Because of the intermittent circuit, the engine will run one minute and not the next. An open in the circuit would result in no signal and a no-start condition. The PCM would not get the signal. A faulty sensor will result in the same stalling and no-start condition. Make sure the sensor's clip snaps in correctly.
These sensors are usually located behind the harmonic balancer (sometimes on the block sensing a ring on the crankshaft or flywheel) and can become contaminated from hot engine oil or coolant. Inspect the ring and check the waveform with a scanner or oscilloscope to make sure amplitude and frequency remain consistent.
Use an ohmmeter to check the sensor's resistance compared to specifications. The symptoms of a faulty crankshaft position sensor include no-start, intermittent stalling, and poor engine performance. This is because this sensor affects necessary fuel and ignition timing.