Check Engine Light

First you have to understand that diagnostic fault codes never say to replace parts or that they're bad, so forget number 3. Next, it is very unlikely a magic "mechanic-in-a-can" is going to fix a mechanical problem, so forget number 1 and 2.

Fault codes only indicate the circuit or system that needs further diagnosis. About half of the time there is some other cause besides the sensor referenced in the code. There could be a broken wire, a corroded splice, or a corroded terminal in an electrical connector. There is no fault code that indicates a vacuum leak. The vacuum leak is one possible cause of setting a code but it's that cause that needs to be diagnosed. It sounds like they're giving you the diagnosis after just reading the codes. Mechanics read the same codes, do the necessary diagnostics, THEN determine the solutions. Your parts guys skipped a step.

You need to get the exact code numbers or descriptions, then we can suggest some tests to narrow down the cause of the problem. You can go here:

https://www.2carpros.com/trouble_codes/obd2/P0100

to get an idea of what the typical descriptions look like.