Diagnosing what is wrong with a car involves troubleshooting based on what the problem looks like, feels like, sounds like or smells like. Most DIY diagnostics on the web such as Auto MD or Auto Zone start with these symptom checkers.
A DIY diagnostic leads the car owner through a list of symptoms, similar to a medical diagnostic system. A click on each symptom brings the car owner to a narrower list until it is down to a few different possibilities. For example, using Auto Zone's troubleshooter, the owner can click on poor fuel economy as a symptom under the look category. That brings up one option; during refueling is likely when the owner noticed the poor fuel economy. The next options are the engine and rear of vehicle. Clicking on engine brings up a list of possibilities including bad spark plugs, dirty fuel injectors, problems with the fuel pump or faulty radiator hoses. Through a process of elimination, the owner can figure out what may be causing the car to have poor fuel economy.
General best practices on diagnosing a car problem include writing down the suspicious sound, feel, smell or appearance, as well as how often it happens, when it happens and what else might be happening at the same time.