If your car or truck is under warranty, then a trip back to the dealer or your favorite mechanic is in order because the company that made the car is likely to be footing the bill.

But if you are off warranty, the choices and problems get more complicated.

Mechanics are a lot like doctors. They perform more tests or suggest fixes out of a two-headed motivation: a wish to run up a bill and to protect themselves against liability.

That's why as vehicle owners you need to do a little homework and advance work before you go to a mechanic, even one you love, to avoid paying more than you should or being taken advantage of.

Before we start, it is best to keep a log of your repairs in your owners manual, which often has blank pages to keep track. That way, you will know for sure if your air filter was changed 6,000 miles ago when you had your last oil change before the mechanic offers to sell you a new one.

1. Beware the simple over-charging

1. Beware the simple over-charging

I hate to say this, but decades of incident reports filed with the Better Business Bureau indicates that a mechanic, auto body or fix-it shop is more likely to pad a bill when the customer is a woman or elderly. We have seen advice given on the Internet by some of our competitors at AOL Autos that actually suggests to women they take a man with them to shop. We would never be so patronizing to our women readers. Instead, we advise that you always try and get two written estimates for the work to be performed--including itemized parts and labor.

When assessing the estimates make sure you are looking at apples versus apples; that both places are using "official" automaker parts, such as GM parts or BMW parts, not knockoffs or generics.

2. Beware the hard-sell to replace air and cabin filters

2. Beware the hard-sell to replace air and cabin filters

This is especially the case at quick-lube shops. I'm fairly convinced that these places have one set of really dirty filters that they show everyone. When we get our oil changed at such places, we actually watch the engine compartment and service tech for the time in between he opens the hood and somebody shows us a dirty filter to see if it actually came out of our car. City drivers are advised to change filters every 15,000 to 25,000 miles. It may be more often if you drive a lot of dirt roads.

3. Beware the worn brake pad replacements

3. Beware the worn brake pad replacements

Brake pads need to be replaced when they get down to about 1/4 inch. If it is being suggested that you replace yours, ask to see a pad that has not been taken off yet and shown to you. Just because your brakes are squealing a bit does not indicate that you need new pads. Often, brakes squeal after a car has been sitting for a bit. Excessive brake dust, a bit of rust forming on rotars during rainy or damp periods can cause squeal.

It's hard to say just how long a set of brake pads should last, but automakers agree on somewhere between 30,000 and 70,000, a big range we know, but it depends on how aggressive the driver is, the make of car and brake pad, etc.

4. Beware the coolant flush

4. Beware the coolant flush

What could sound better than getting a good flushing out? "Hey bud..your coolant is looking a little muddy….could use a good flushing." Unless your manufacturer recommends a flushing, it's a waste of your hard earned money. The coolant, which can be topped up if need be, is designed in some cases to stay in the car for five years or 150,000 miles.

And you have to be mighty careful about topping it up. Let's say you have a brand of coolant like Dex-Cool, which is orange in color. This is a long-lasting coolant preferred by companies that buy cars in fleets, like state governments and pharmaceutical companies. If someone adds green coolant, which ethylene glycol based, your orange coolant will be degraded from lasting 150,000 miles to lasting about 30,000 miles. If you bought your car used, it could have been a fleet car.

If you start out with green anti-freeze, then change it every 30,000 miles. An actual flush of the system is only recommended every 50,000-60,000 miles.

5. Beware plugs and belt replacement

5. Beware plugs and belt replacement

It's not that these parts don't need replacing. It's that mechanic shops recommend replacing them more often than is generally needed. They are easy jobs to do, and mark up. You should look at your own belts before taking the car in for service. Are they showing cracks? Are they taut? If the answers are No and Yes, then don't go for it.

Before agreeing to have plugs changed, check your mileage and hopefully the log you have been keeping about maintenance and part replacement. If your car seems to be not performing up to snuff, or its consuming more gas than normal, you could need new plugs. It can also mean dirty fuel injectors.

Spark plugs in a car need replacement every 30,000 to 40,000 miles, depending on your car. Some luxury cars have plugs designed to go 100,000 miles. So, consult your manual about your car, and keep track of those repairs. Can you remember 15,000 miles later if you authorized new plugs to be installed? Write it down. And save your money.