There a number of mechanics who take profit of people's ignorance of car mechanics to charge for unneeded or even non-existent services, like change the blinker fluid or perform an capacitive discharge. What crime such mechanic would commit if he/she does this? Is that a fraud like any other or are there any specifics?
No crime is committed if a person performs a service and ineptly describes the service. To change the context a bit, I might contract with a guy to build a wall and he says he will charge me for installing a "Swedish drain" when in fact what he will install is called a "French drain". If he installs the thing, it does not matter (legally) whether he calls it by the conventional name. I am not relying on the distinction between French and "Swedish" drains, and that is not material. However: he may specify that the drain will use 18 inches of 1.5" drain rock, but he uses (and intends to use) 18 mm of 3/8" crushed rock, and that is a material fact. In the latter case, he has committed fraud.
The same considerations go into dealing with "unnecessary" service, which however is more about "what he said". Let's assume that you come in with a flat tire and the mechanic offers to overhaul the engine. If you agree to this service, that is not fraud, because he did not say something false that you depended on. If, however, you ask "Why would overhauling the engine be necessary" and he says, I dunno, "Because by law, I can't repair a tire without first overhauling the engine", or "Because you flat was caused by astral radiation from a poorly-tuned engine", then that would be fraud – the statements are false, and you relied to their truth, in agreeing to the service. On the third hand, reasonable statements like "it might help", "it could work" are not deceptive, even if it turns out they are not true. Fraud is not about statements that "turn out not to be supported by the facts", it is about statements that you know to be false.