Did you know that the US introduced legislation many years ago that mandated that car owners should be able to diagnose faults on their own cars?
The result is that pretty much any car manufactured in 2001 or later has an "OBD2" port fitted (within 600mm of the steering wheel in theory). The result is, you can now get a cheap (< £10 in the uk on Amazon) device, which plugs into the port, and sends fault codes and a range of performance data by bluetooth back to your phone.
What this means is if you like to do your own servicing and repairs, you should be able to. More to the point for most people, you can see the fault code for yourself and figure out if your garage is telling you the truth about the reported faults. You might even be able to cut some of the diagnostic time off your garage bills.