Had to choose between a 3-hour diagnostic or just replacing the alternator on a 2008 Civic
I went with the diagnostic, found it was just a corroded ground wire, and saved the customer almost $400, but man, I really had to argue with the service writer who just wanted the quick sale.
That service writer probably sees a dozen Civics a week with bad alternators. It's an easy guess that's usually right. Doubt they were too pressed about missing one sale.
Yeah, that "salty" look is real. I've had to pull service writers aside before and just lay it out. You gotta tell them that doing the actual diagnostic builds way more trust with the customer than pushing the easy part. That customer will come back and tell their friends. Missing one alternator sale is nothing compared to that.