Do different shops.... Well, YES.
Some shops have a bunch of 20 something kids just out of grease monkey school who basically have a problem telling the difference between a piston and a valve.
Some shops have a 50 something gruff old guy who has been working on motors since he souped up his go-cart back in his single digit age days and who can tell what's wrong with your engine just by the sound it makes as you pull into the lot.
Ok, I exaggerate a bit, but that is not far from the truth.
When my 8.1 had that same kind of problem no codes were set by the ECC, the Check Light did not come on.. But the 50 something's gruff old guy found it in less than 10 minutes.