Mechanics are people too. We like to be treated with respect and the majority of us want to help you when something goes wrong with your car. We also want to help prevent future problems whenever possible. To that end we will suggest regular maintenance items to be performed on your car. When customers bring their vehicles in for repair we hear a lot of "my brother says I need a tune up" or "I need an alignment" etc. The list goes on. Customers who come in and ask for a tune up when what they really meant was an oil change. This leaves a lot of room for mistakes and misunderstandings.
If you are experiencing something unusual with your car, make notes on what is happening and when. Then when you bring it in explain what you are experiencing. An example would be something like this. "I can feel a vibration in the steering wheel between 55 and 70 mph". I can not tell you how often customers come in saying they need an alignment instead of giving us this little piece of information. Alignments do not cause this, although an out of balance tire may. I always ask my customers why they want something done before doing the repair they ask for but, not all shops do. can you really blame your mechanic if they do what you ask for and the problem still exists? Let your mechanic do the diagnosis, give as much information as you can on what you are experiencing and things will go a lot smoother.
When you bring your car to a repair shop, please try to understand that multiple calls asking "is it done yet" or "did you find the problem?" is very frustrating and often takes someone away from other work. Maybe even the person working on your car. Ask when you leave your car when there would be a good time to call or just wait for the shop to call you. Most of us know how important you car is to you and cars never break at a convenient time.
It is not an insult to a good reputable shop to ask for your old parts and is your legal right in most states. However asking for them after the job is done is not acceptable. If you want your old parts the time to ask is when you first drop off your car. Be aware that some re-manufactured parts have a core charge and must be returned to the parts supplier, if you want to take those parts with you, you will have to pay the core charge. In that case just ask to see the parts. None of this should be necessary if you have a good relationship with your mechanic.
A new thing in recent years is the auto parts store offering to scan your car for free when that pesky check engine light comes on. Then the customer either buys the part they say is bad and puts it on themselves or worse takes it to the mechanic and ask him to install it. The most common one I see is the O2 or oxygen sensor. This is wrong in so many ways. You just got what you paid for, you paid nothing and that's what you got. It's rare that the parts guy is a mechanic and there is a good chance they are wrong. Next you would not take a steak to a restaurant and ask them to cook it, why take away our profit? Here is how these works, we buy the part we know to be reliable and pay a little less than you for the same part. We now stand behind the part and workmanship. And please, if you do have a parts store or neighbor scan your car do not let them clear the code. In most modern cars there is a lot of information available to us that the cheaper scan tools and code readers do not have access to. one of which is freeze frame data. when your car set the code it probably recorded all of the sensor data in that moment and could give your mechanic something to work with, when you cleared the code you also cleared that data.
In closing, try to understand that mechanics are only human, we make mistakes. If you feel something was not done right talk to your mechanic as a human being and leave the anger at home. If you have found a good mechanic he will correct any mistakes and if no mistake was made he will show you and give an explanation. If they are unwilling to do this, then you need to find another shop.