When your car is acting up in any way, your mechanic will probably put it through a number of diagnostic tests, looking for the exact cause of the problem. Their equipment and their experience can quickly tell them exactly what's wrong with the car, but it's good if you communicate with them certain details about the problem as well. Note a few of those details you'll want to share, and how this can ensure you get an accurate diagnosis.
Secondary problems with the car
Secondary problems refers to other issues you're having with the car, beyond the reason you're bringing it into the shop. Sharing these other issues with your mechanic may help him or her to narrow down the reasons why the car is acting up.
As an example, if the car sometimes struggles to start, note if it also seems to sputter while you're driving, or if you notice any odd smells coming from under the hood. This can tell your mechanic if the car needs a new oxygen boot, which delivers oxygen to the engine so it can mix with fuel and create combustion, rather than needing a new fuel pump or battery. If you notice any of these types of seemingly minor problems with the car, don't think they're unimportant, but mention them to your mechanic when you bring your car in for service.
Don't be embarrassed to tell your mechanic about any DIY fixes you tried on the car before you brought it in for repairs. They may need to work around those fixes that may not be very secure and durable, such as an impromptu tape job that's holding together exhaust pipes. A mechanic will also need to know what has been done to try to address the problem you're having with your car, so he or she can rule out certain potential repairs.
If you seem to have the same problem with your car over and over again, this should also be noted to the mechanic. This is because the problem is probably not being properly addressed; for example, if you've had the alternator replaced because the car's battery keep running down, the car may need new wiring, rather than another alternator. By telling your mechanic what issues keep recurring and what fixes you keep trying, they can know what to rule out by way of a potential solution, and they won't waste time trying the same fix again.