Beware of the check-engine light. It's another profit center for a lot of dealerships and garages. The check-engine light is a sensor that is telling you that something is amiss in the car. It doesn't mean the car will self-destruct or die suddenly on a highway.

"Probably the most common cause of the check-engine light is that the gas cap is not on tight enough," Reed said. The sensor has responded to the extra oxygen going through the gas line and it will go off once the cap has been tightened or the entire tank has been used.

Many mechanics will offer free diagnostic tests to tell you why the check-engine light is on. Consider that another red flag.

"Mechanics can tell you anything once the check-engine light comes on," Reed said.

Ask the mechanic to show you the problems. If the transmission fluid is not pink, but a dark brown, it's time to change it. If it's gritty because of accumulated pieces of metal and plastic, changing it could cause the transmission to slip further. If the dip stick for the transmission smells like barbeque sauce, then there are problems.

"Don't be afraid to ask them to explain these things to you," Davis said.

Keep a precise record of repairs and check them before you bring the car in. Schaffels said she used to keep a log of every tank of gas she purchased, where the mileage stood, what she paid for the gas and how the fuel economy tracked.

"If your vehicle's fuel economy has changed, it tells you that something needs to be adjusted," she said. It could be as simple as putting air in the tires or replacing rotors or plugs.

Click here to become a fan of MSN Money on Facebook

Know your car. Listen to how it starts and stops, how the wheels and brakes sound when you turn corners or come to quick stops. How does the motor hum? Does it rattle anywhere? Know how your car sounds when it's running well so you can recognize that it sounds bad when it's not.

"If you know what it sounds like when it's not running well, that makes it easier for the mechanic to fix the problem," Schaffels said.

Communicate well. The worst thing you could do is throw the keys on the counter and tell the mechanic to figure out what is wrong with the car. Let him know what the vehicle's problem is, the sound you're hearing and where it's coming from.

"Give the mechanic enough information about the problem so that he's not spending hours and your money to figure the thing out," Davis said. "You don't walk into your dentist and say, 'I'm pretty sure it's that tooth. Just go ahead and pull it.'"

Be nice. Your car breaks down on your way to work the day of the huge presentation. You're angry and desperate. It's not the mechanic's fault, so don't direct your frustration at him.

It could end up costing you. Davis' father charged a 10% penalty premium for customers who were rude to him or his employees because of car breakdowns.