"Lemon" is the generally accepted term for a defective car or a car with recurring mechanical problems that interfere with use. While many experts say that contemporary car manufacturing quality standards make it less likely that you would buy a new lemon, it's certainly not out of the question. Edmunds has a good article on what to do if you think you've been stuck with a lemon.
Consumers are in luck today with the wealth or resources online. First, research can and should start in the buying process because preventing problems is always better than dealing with them after the fact. Buyers can research car reviews, dealerships, consumer complaints. For used cars, buyers should do a VINCheck and a vehicle history search and in the case of used cars. (See our post on avoiding flood damaged cars).
If problems do occur after purchase, your warranty and your dealer is the first place to turn. Document your attempts to have things fixed, including any out-of-pocket costs and time that are involved. If problems continue, it's much easier to research things online today to see if your problem is common and to check with manufacturers. There are also a variety of ways to learn about vehicle recalls. But if all else fails, most states offer some type of consumer remedies under what is known as "lemon laws."
State laws vary as to whether they cover both new and used cars, and most require that the car was purchased in that state and with a warranty. Cars that are purchased under an "as is" agreement would generally not be covered. Plus, states have various gating issues before any remedies would kick in: the buyer must have tried to resolve the issue in various ways before being eligible for consumer protection. According to Edmunds:
State laws vary in what constitutes a "persistent" problem or the "reasonable" number of repair attempts that would get you over the border into lemon territory. In Connecticut and New York, for example, four repair attempts is the state standard for "reasonable," according to Connecticut attorney Sergei Lemberg, whose site, Lemon Justice, can help determine if you've got a lemon. But in Massachusetts, the law requires three attempts to repair the same problem in the first 15,000 miles -- and one last attempt to get the manufacturer to address the defect after that.
Here are links to state lemon laws for New England states.