Transcript for Confessions of a Car Mechanic
What really happens when you take your car into the shop? Gio Benitez with some costly repairs you may not need. Reporter: Stephanie, an ABC news producer, takes her infinite to two highly-rated mechanics to ensure her vehicle is in perfect working order. This car is road ready. Reporter: Ace mechanic Audra fortin rigs it so the check engine light, known by some as the idiot light, will be on. But a diagnostic test, which costs $50 to $100, should quickly detect the problem. Stephanie goes to this repair shop, Monty's in north babylon, New York. My service engine light's on. Oh, okay. Referee: Our hidden cameras show a mechanic fixing the actual problem seconds after opening the hood, by plugging that loose wire back in. But another mechanic suggests he can take the sensor apart and fix it for $190. And $190, I can remedy your problem. Reporter: When we return to pick up the vehicle, the mechanic at Monty's proudly shows Stephanie their work. So what I do, I open it and go one by one, clean it, you know. Reporter: When we check our hidden cameras, we discover that Monty's never even touched that sensor they said they would take apart. We did get a quick spray and rinse of the engine. And, of course, that loose wire was plugged in. Gio Benitez with "20/20." You said you took apart that sensor, and you actually didn't do anything. I don't know what you're talking about. Reporter: You don't? No. Reporter: You want to see what the video says? He says another mechanic did the work. That mechanic said there must have been a break in communication. How can you protect yourself? Know your car.
This transcript has been automatically generated and may not be 100% accurate.