First Do Your Homework
Before you go shopping, research vehicle reliability reports online for the different models you are interested in. Both Consumer Reports and Edmunds publish owner surveys and reviews and are good sources for finding out about troublesome vehicles. Other sources exist too. Look to see what others say is the best vehicle in the type you are interested in most. Also look to see what others say is the worst. Before you ever go shopping, on the lot or online, decide what type of vehicle you want and what models you will look at.
Check Recalls and Known Defects Records
If you want to know more about that car you're considering, then check all four of the federal government's databases (http://www-odi.nhtsa.dot.gov) for any recalls or service bulletins or safety investigations and owner complaints on the model vehicle you are interested in. Service bulletins are notices that the manufacturer sends out to their dealerships to warn them about problems that have been discovered in particular model vehicles and how to try and fix them. The only way to know if your vehicle was repaired for the problem already is to check with your local dealer's service department and get a vehicle repair history from them.
Read the Buyer Guide on the Window
Federal law requires every used car on any dealer's lot to have posted in the window this one page form that clearly discloses whether the car is being sold "as is" or with a warranty. The car dealer is required to properly fill it out, front and back, and to give a copy of the form to the buyer in the sale also. Make sure you understand what guarantees you get with the car and make sure all promises are written down on the sales contract or on this Buyer Guide form. When a motor vehicle is sold "as is" it could mean that you have no recourse against the selling dealership if problems or defects arise later. Generally, cars with problems are the ones that are often sold "as is" so be careful. You should always try to get the seller to give you some kind of written guarantee, even if it is only good for 30 days. You can see what the form looks like at this web site page: http://www.ohiolemonlaw.com/pdf/Used-Car-Window-Sticker-Form.pdf
Ask the Seller
Find out everything you can about their experience with the car. Ask them for any repair records they have, so you can see for yourself. If the owner doesn't keep repair records, ask them where they had most of the work done on the vehicle and go ask the repair shop. If you aren't sure, write down the vehicle's serial number and go to the nearest new car dealer's service department and ask for the vehicle's repair history printout from the factory computer. You may have to talk them into doing it but practically every new car dealer has computer access to the factory records that can show most of the repair work done on the car by any factory-authorized dealer. Repair shops do not legally have to voluntarily give you this vehicle information, however.
Inspect the Vehicle Thoroughly
Check the outside, inside, engine compartment, tires, steering, suspension ride, and look for colored exhaust smoke. Outside, look for body paint or panels that don't match up, which can be a sign of body repairs. Inside, make sure all the warning lights work (they should light up when you start the car and go out once the car is running) and make sure the warning lights don't stay on. Check the carpeting everywhere for mildew which can mean water leaks or a flood car. In the engine, look for fluid leaks, low fluid levels or unusual colors. Metal particles, tiny lumps, burnt odor or a frothy residue can mean engine damage. Billowing white tailpipe smoke could mean engine repairs. Blue smoke can mean its burning oil and expensive repairs. For tires, a worn spot in the middle of the tire often means an overinflated tire. Lots of wear on a tire's outside edge can mean a lot of hard driving and fast cornering. Uneven cupping tire wear can mean suspension or brake problems.
Get a Professional Inspection
Before you buy it, take the vehicle to a local mechanic to have it thoroughly gone over. It'll probably cost you some but it's worth it to know for sure what problems you can expect to come up during your ownership. AAA also may recommend a repair shop or you can go to your nearest Firestone or Goodyear dealer or another independent repair shop and ask for an inspection.
Get an Online Vehicle History Report
If you know the vehicle's serial number (called a VIN number), you can get a report online that can tell you where the vehicle has been owned, if it has been titled with a salvage or flood title or other checkered past. CarFax and AutoCheck are two web sites that gather data from insurance companies (including accident records) and public title department records and there's also NMVTIS. For a small fee they will give you a printout that describes most, if not all, of the vehicle's history for a very low cost. Every vehicle's VIN number is located either on the door jamb or in the windshield area of the dash on the driver side and every digit means something. To understand what your vehicle's VIN numbers mean, you can find VIN number decoders on many free internet web sites.