You’re better off assuming your customers are car experts rather than car-ignorant. One shop manager learned this valuable lesson from a little old lady who knew her vehicle all too well.
It was all due to an unfortunate misunderstanding — three elderly ladies standing in front of my desk accusing me of overcharging for a repair and giving me an earful. (Not something I’d ever like to repeat.) But I could’ve avoided the situation entirely and saved my customer grief if only I’d communicated better.