John DeCostanza might as well be marketing ClearMechanic in his recent piece for AllCarAdvice. He even cites the example of a thermostat job, one we often refer to as commonly misunderstood yet easy to explain (esp. with visuals). DeCostanza’s insight is that a lack of customer education is considered a proxy for untrustworthiness.
So the explanation should not be anything too technical or weighed down in automotive jargon and certainly shouldn’t be condescending either. Remember that in something that can seem as foreign as auto repair, knowledge is power and if it seems like the repairer is trying to limit your knowledge it can be a sign of his reluctance to be truthful and transparent.