When you go to a mechanic to get a diagnosis of your car problem, the repair shop may suggest unnecessary repairs that will cost you more money. That's why it's good to learn everything you can about your car and then try to diagnose the problem online first. You can then go to your mechanic as a knowledgeable consumer.
Learn about cars online first. Before you can accurately diagnose and understand you car problem, you must understand the inner workings of cars.
Find out how car engines run, what purpose various parts serve, how a car starts, how fuel is injected through the system to keep the car running, purpose and function of the exhaust system and transmission, and all about general car maintenance (oil changes, spark plugs, belts, fluids, coolant, batteries, brakes, and wheels). See "Resources" for a site where you can study this information.
Make a list of the specific problems you are experiencing with the car---for instance, a noise (and from where) or start-up problem.
Browse mechanics forums that are dedicated to your car's make (for instance, ToyotaNation.com). There is usually a specific category dedicated to each model (for instance, Camry, Corolla and Tercel).
Look at existing threads from people who seem to be having the same types of problems. Register and post information about your specific problem in a new thread if you don't see a clear answer. Make friends with the mechanics on the site, and don't challenge their expertise. Simply listen to suggestions and take notes.
Isolate the areas of the car that could potentially be the cause of the problem based on mechanics' suggestions. The mechanics and other car experts will likely suggest tests and checks that you can perform on the car to determine if their theories are correct. Perform the checks and report back to the forum to get a preliminary diagnosis.
Ask a mechanic directly using a paid service such as AllExperts.com or 2CarPros.com as an alternative. Choose an expert from the "Auto" category. You will either have to bid on an answer or pay the expert's posted rate.
Don't always automatically assume the worst diagnosis is true for your car. In addition to the online diagnosis, get a second and third opinion from actual mechanics. Go to your local auto-parts store to ask if it offers free car diagnostic reports. This can save you £32 to £65 at a mechanic. AutoZone is one store that does free diagnostic checks.