The PCM has "Learning" capability that allows it to make corrections for minor variations to improve driveability.
If the car battery or PCM is disconected, the "learning" process resets and begins all over again.
The PCM can relearn operating parameters in a short driving period.
However, if the PCM memory is lost and the car is not driven ( to relearn) prior to returning to the customer, the customer may notice a change in car Performance for a short driving distance.
Note: Anytime the PCM losses power, the I/M Readiness Flags (software) will be rest to OFF.
To reset the flags, drive the car through a FULL OBD-II drive cycle

Crank (CKP) Sensor Relearn Process
Requires a PCM scanner to command the PCM to enable CKP system variation learn fuel cutoff and allow the crankshaft position system variation compensating values to be stored in the PCM.
The PCM must detect an engine speed of 5,150 RPM during the Crankshaft Positon System variation learn procedure in order to store the crankshaft position system variation compensating values and complete the procedure.

NOTE: By not doing a valid CKP test, engine misfires can occur until crank positon is known by the PCM.