Since the introduction of OBD, or "on board diagnostics" in the 1970s, warning lights have meant more than just telling you there's a problem. Onboard computers receive and store codes that help a mechanic determine what is wrong.
As OBD systems improve, the information they store becomes more detailed. "The system can store 7,000 separate codes," said John Nielsen, AAA National Director of Auto Repair and Buying. The plug-in scanner reads those codes, and each one pertains to a part of the vehicle.
Here are some key warning lights:
• The tire pressure light (usually a tire symbol with an exclamation point) comes on when one or more tires are significantly underinflated, Nielsen said. This may be the result of gradual pressure leakage over time or a puncture. If the tire pressure monitoring system displays individual tire pressures, check the dash display: If the pressures are all within a few pounds of one another, the warning is probably from gradual leakage. Check and adjust the tire pressures as soon as possible.
If one pressure is significantly lower than the others, a puncture is likely. Some vehicles just have a general tire monitoring light, and it doesn't provide specific information for each tire. Look for one tire that appears lower on air than the others. Be aware, though, that these monitors can malfunction, especially in cold weather.
• The oil pressure light (usually an oil can symbol or the word "OIL") illuminates when there is a drop in engine oil pressure, Nielsen said, and you should take it very seriously. Of all the warning lights, the oil pressure light indicates the greatest potential for serious mechanical damage. Drive slowly to a safe place, pull over and call for help.
• The engine temperature light (usually a thermometer symbol or the word "TEMP") comes on when the engine temperature has exceeded the safe maximum. If the increase in temperature is not stopped, major engine damage can occur. If there are no other overt signs of overheating, such as steam or liquid coolant coming from under the hood, roll down the windows and turn on the heater full blast as you drive at a reduced speed. If the warning light stays on after a couple of miles, pull off the road at the first safe opportunity and call for help.
• The charging system light (usually a battery symbol or the word "ALT") illuminates when the vehicle electrical system is no longer being supplied power by the alternator. It's serious, but driving the vehicle to a safe place usually won't cause more damage.
• The check engine light (usually an engine symbol with the words "CHECK ENGINE SOON") typically comes on when there is a problem affecting the vehicle's exhaust emissions. If the check engine light comes on and stays on, make an appointment with an auto repair shop to have the problem checked as soon as possible. But if the check engine light begins flashing repeatedly, the catalytic converter may be overheating, and it could be dangerous to keep driving.
