The inline six cylinder engine in my 2002 GMC Envoy with 80,000 miles suddenly began to run rough and the check engine light came on.  I thought it might be a bad tank of fuel or a loose fuel cap.  The light would remain lit at slow speeds, and at highway speeds it would flash.  The problem persisted after a tank of highest quality fuel was consumed with a tight fuel cap.  I purchased an Actron AutoScanner from Amazon.com.  The scanner informed me that cylinder one was misfiring.  I knew the original spark plugs should be replaced.  Each cylinder has its own ignition coil.  The ignition coils need to be removed to install the plugs below.  Each coil is attached by only one bolt.  While removing all of the coils I marked the cylinder one coil.  I replaced all of the ordinary worn plugs with premium Bosch Platinum + 4 plugs.  I installed the coil from cylinder one on cylinder two and the other coils I installed randomly on the other five cylinders.  I started the engine and it did run better due to the new plugs.  The engine still ran rough however and the check engine light did remain lit.  The Actron AutoScanner then informed me that cylinder two was now misfiring.  Bingo!  I purchased a new ignition coil from Federated/Ficher Auto Parts for $60.  The GM dealer price fore the same part was $130.  I bolted on the new coil and my troubles were over.  The Actron AutoScanner performed excellently.  The scanner protected my small bank account from the dealership service department.  The scanner minimized the time I spent diagnosing the issue.  I stored it carefully away for the next time.