Symptoms inlcude odd stuff that you can't explain.
Like getting a misfire code after doing nothing but a flash.
Odd misfires that you can't determine, no matter the amount of testing etc.
I've had a few that after a motor/trans swap etc that upon startup run like crap and after checking things over you are left with no leads. Crank learn is like waving a magic wand and all woes are fixed. The issue is basically that the pcm doesn't learn the exact crank position and therefore is not tossing the spark etc at the correct times.
BrandonHall recently had some odd behavior from his Regal and no matter what he tried nothing quite fixed it. Crank learn and it's a happy car once again.
