An old oxygen sensor (say, 90,000 miles or more) may still work well enough that it doesn’t trigger the check engine light but could still hurt fuel economy. Engine performance can also be reduced by more serious internal problems, such as valves that don’t seat properly or worn piston rings, or by restrictions in the exhaust system. Because the same symptoms can suggest different problems, and there are often several possible causes and cures, it’s better to consult a professional mechanic than to try to be one if you have neither the experience nor the right equipment to diagnose drivability problems. In short, rather than ask for a tune-up, tell a mechanic what you’re experiencing and ask him or her to find the cause.
You can speed the tune-up process by being ready to describe what happens and when (such as whether your car hesitates when the engine is cold or when passing at highway speeds), any sounds you hear and what you feel when your car’s "illness" shows up. One caution about lower fuel economy: You should expect it to go down at least a little during the cold months, and maybe a lot. Colder temperatures make your engine and charging system work harder. In addition, winter gasoline blends have slightly less energy content than summer blends, so they don’t deliver as many miles per gallon. A tune-up won’t make Old Man Winter, or his effects, go away.
Vehicle charging systems haven't changed much over the last few decades. But with the increasing complexity of the modern automobile, along with the addition of many aftermarket accessories such as cell phones, DVD players, high-power audio systems, alarms etc., the role of the charging system is now more important than ever. The charging system on your vehicle can be thought of as a small power plant which does exactly what its name implies - charges and maintains a good state of charge on the battery.
If belt tension is correct, then the next item to check is the alternator itself. You now have two choices - you can test the alternator yourself with a simple test using a voltmeter, or you can have the test performed by a professional. If you choose to do the test yourself, then you will need a DC voltmeter, or a multimeter. Such a unit can be purchased at most auto supply stores, hardware stores, department stores, home improvement stores, etc.