What does it mean?
OBD II fault code P0321 is a generic code that is defined as “Crankshaft position (CKP) sensor/engine speed (RPM) sensor range/performance problem”, and is set when the PCM (Powertrain Control Module) detects an input signal from the crankshaft position (CKP) sensor/engine speed (RPM) sensor that is erratic, implausible, or falls outside of expected parameters.
NOTE #1: Note that on older applications that still use distributors, this code may be displayed as P0321 – “Ignition/Distributor Engine Speed Input Circuit Range / Performance”, which means exactly the same as the definition given above. NOTE #2: On VAG group applications, this code may be displayed as “16705/P0321/000801 – Engine Speed Sensor (G28): Implausible Signal”, which means exactly the same as the two other definitions given above.
NOTE #3: On most modern applications, the Crankshaft position (CKP) sensor does double duty, and is used to monitor the engine speed in addition to providing input data on the position of the crankshaft to the PCM for the purposes of calculating ignition timing and fuel delivery strategies. The primary purpose of the CKP (crankshaft position) sensor is to provide precise input data to the PCM regarding the rotational angle of the crankshaft.
Without this information, the PCM is not able to calculate an appropriate ignition timing, and hence, an appropriate fuel delivery strategy. In practice, most applications use a reluctor ring (that is attached to the crankshaft), and (most commonly) a Hall-effect sensor whose induced magnetic field is interrupted by the teeth on the reluctor ring as the ring rotates.
Since the teeth on the reluctor ring are evenly spaced, the PCM is able to “count” the number of times the sensor’s magnetic field is interrupted, which information is used to both calculate the engine speed, and the position of the crankshaft. However, all reluctor rings have a spot where one tooth is “missing”, thus creating a gap that is twice as wide as the gaps between all other teeth.
This gap however, is located relative to piston #1, which means that the gap serves to indicate to the PCM where piston #1 is in its upward motion during the compression stroke.
In practical terms, the regular on/off square waveform pattern that is created when the reluctor ring rotates is “broken” when the wider gap in the reluctor ring’s teeth passes in front of the CKP, thus creating a reference point during each revolution of the engine that the PCM uses to calculate ignition timing and fuel delivery strategies.
While the above is a gross over simplification of a very complex process, it should serve to illustrate the point that without accurate input data regarding the position of the reference point on the reluctor ring i.e., the position of the crankshaft relative to piston #1, the PCM is unable to determine when to deliver a spark and fuel not only to cylinder #1, but to all cylinders.
Thus, when the PCM detects an input voltage from the CPS that falls outside of expected parameters, or is abnormal in any way, it will recognize that it cannot control the ignition and fuel delivery systems effectively, and it will set code P0321 as a result. Note that whether or not a warning light is illuminated when code P0321 sets depends on both the application and the severity of the problem.