Be wary of advertisements
Do you remember the old saying, "If it's too good to be true, it usually is"? The newspaper classifieds and mass-mail coupons are commonly stacked with places offering specials to fix brakes, transmissions or any other part. However, don't let your guard down so quickly. The special may not be work your car needs. What's more, the mechanic may be so focused on giving you what the ad specifically says that he may not check other vital components.

Read the owner's manual
Yes, you should read that dust-covered book that's stashed at the bottom of your glove compartment. It explains when to replace most engine parts. Many auto shops and dealers try to sell customers services they may not need. If you don't know when those parts need inspection or replacement, you'll take the bait.

Deal with a qualified mechanic
Read the certificates hanging on the wall, and if there aren't any, you should worry. Look for the Automotive Service Excellence Blue Seal, which indicates technicians' competence in areas such as brake work, engine repair and alignment. A shop gets the ASE designation when 75 percent of their technicians are certified in one or more areas of repair work. Go to the ASE Web site to locate shops in your area with the seal.

Having the seal doesn't guarantee that a mechanic is honest, but at least he knows what he's doing.

"I would like to see consumers more aware of the car they're driving," our auto expert says. "Educating yourself is very important."

Women beware
Another source who was a mechanic for 24 years adds a special warning to female customers.

"Mechanics take advantage of women -- that's standard," our source says. "That's because women are more inclined to believe you."

Women are especially vulnerable to being sold new parts they don't need replaced. Our second insider says this is the most common questionable practice he saw mechanics pull on customers, both women and men.

"Sometimes a shop is having a promotion to sell or move car parts," our source explains. "They'll sell you stuff you don't really need. I've seen guys sell tires that way.

"They use fear, raising safety issues -- especially with female customers."

Our source says it's rare to see a customer charged for a part that isn't replaced. However, it is common to be sold parts you don't really need.

Most or least
The source says that when a car needs a repair, the customer wants the least amount of work done to fix it, while a mechanic wants to do the maximum. In this case, the mechanic isn't necessarily trying to rip off the customer. He just doesn't want something related to the problem to break a week later and then he has to fix it for free!

"It's a genuine disagreement," the source says. "One viewpoint is: 'if it ain't broke, don't fix it.' The other is 'Sometimes doing the most expensive thing is, in the long run, the cheapest thing.'"

Mechanics usually win that argument.

"There's a gap in knowledge," the source says. "The mechanic knows what's going on and the customer usually doesn't. Mechanics have an advantage, and they use it to take advantage of people."

Keep in mind that mechanics don't know everything; sometimes they'll replace the wrong part thinking it will fix the problem. When it doesn't, the mechanic has to find the part that's really broken. And guess who pays for the extra parts and time?

"In that case, a mechanic will tell a customer, "Look, when I was down there, I saw that the so-and-so was in terrible shape, and it needed to be fixed, too," the source says.

Watch the dealer
All cars are not repaired equally, at least at dealerships. When a car is under warranty, the manufacturer sets the fee for the repair, our source explains. This means that mechanics make less money for those jobs than they do for non-warranty jobs. So the car ends up being repaired by the least-experienced mechanic. "Newer mechanics get caught with all the grunt work," the source says.

This is why, when you have something on your car fixed under warranty, it might take a few trips to the dealer to get the job done right.

Last bit of advice
Use common sense -- and your gut feeling -- when your car needs repair. "Judge for yourself whether the mechanic is telling the truth," the source says. "If in fact you don't believe him and your car can limp home, limp home."

However, if you break down somewhere, don't expect mechanics to give you a break. "Basically, you're at their mercy," our source says.