Your car starts making a strange noise so you take it to the…

Your car starts making a strange noise so you take it to the mechanic.  The mechanic explains that the sound is being caused by either of two things:  a cheap and simple to install part that is known to wear out frequently on your model of car, or, the engine is at the end of its life and needs replacing.  The mechanic suggests replacing the entire engine. What principle of Scientific Literacy did this mechanic violate?