Learned that same lesson the hard way with my 2004 Silverado. I was throwing parts at a rough idle for two weeks (new plugs, new coils, cleaned the throttle body twice) because my $30 scanner just said "misfire cylinder 3" and nothing else. An old timer at the shop finally plugged in his real scanner, looked at live data for like five minutes, and pointed out my long term fuel trims were maxed out positive on both banks, which meant unmetered air getting in somewhere. Turned out to be a cracked intake gasket, twenty dollar part, and I felt like a complete idiot. Now I won't touch a diagnosis without watching fuel trim and O2 readings first, even if it means borrowing a better scanner from a buddy. That one habit has saved me way more money than the scanner ever cost.