You’re driving along in your car or truck and suddenly a yellow light illuminates on your dash telling you to check or service your engine. If you’re like most car owners, you have little idea about what that light is trying to tell you or how you should react.

Call it the most misunderstood indicator on your dashboard: The check engine light can mean many different things, from a loose gas cap to a seriously misfiring engine.

“It doesn’t mean you have to pull the car over to the side of the road and call a tow truck,” says Dave Cappert of the National Institute for Automotive Service Excellence, an organization that tests and certifies auto technicians and is based in Virginia. “It does mean you should get the car checked out as soon as possible.”

Ignore that warning and you could end up damaging expensive components. It can also be a sign that your car is getting poor fuel economy and emitting higher levels of pollutants.

What the Light Means

The check engine light is part of your car’s so-called onboard diagnostics system. Since the 1980s, computers increasingly have controlled and monitored vehicle performance, regulating such variables as engine speed, fuel mixture, and ignition timing. In modern cars, a computer also tells the automatic transmission when to shift.

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When it finds a problem in the electronic-control system that it can’t correct, a computer turns on a yellow warning indicator labeled “check engine,” “service engine soon,” or “check powertrain.” Or the light may be nothing more than a picture of an engine, perhaps with the word “check.”

In addition to turning on the light—known as the International Check Engine Symbol—the computer stores a “trouble code” in its memory that identifies the source of the problem, such as a malfunctioning sensor or a misfiring engine. The code can be read with an electronic scan tool or a diagnostic computer—standard equipment in auto repair shops. There are also a number of relatively inexpensive code readers that are designed for do-it-yourselfers.

The code will typically just point you in the direction of the problem, and still requires an experienced professional to fully diagnose and repair the issue. With the cost of a shop performing a diagnosis costing around $90, many car owners are turning to the internet to find the cause and remedy. This can work in some instances, but when in doubt, seek professional help.

What to Do About the Check Engine Light

If the check engine light illuminates, it will either blink or remain constantly illuminated, depending on the problem. A blinking light, or in some cars a red light instead of a yellow or orange light, indicates a problem that needs immediate attention. Either way, you should have the vehicle checked by a mechanic. 

In late-model cars, a blinking light usually indicates an engine misfire so severe that unburned fuel is being dumped into the exhaust system, where it can quickly damage the catalytic converter, leading to an expensive repair. If that happens, you should reduce power and have the car or truck looked at as soon as possible.

If the light is steady, the problem is not an emergency, but you should schedule an appointment as soon as possible. Today’s automotive computers often try to compensate when there’s a problem, so you might not notice deterioration in performance, even though your fuel mileage might be suffering and your vehicle emitting unacceptable levels of hydrocarbons and other pollutants.

“The customer is really, in the long run, potentially hurting their pocketbook by leaving that light on and ignoring it,” says Jim Collins, a national training team leader for Ford Motor Co. In some extreme cases, the car’s computer may reduce power for you as it tries to limit the risk of damage.

If the check engine light comes on, here are some tips on what you should do:

  • Look for a serious problem that requires immediate attention. Check your dashboard gauges and lights for indications of low oil pressure or overheating. These conditions mean you should pull over and shut off the engine as soon as you can find a safe place to do so. On some cars, a yellow check engine light means investigate the problem and a red one means stop right now.
  • Try tightening your gas cap. This can often solve the problem. Keep in mind that it may take several trips before the light resets. Some vehicles have a separate indicator that warns of a loose gas cap.
  • Reduce speed and load. If the check engine light is blinking or you notice any serious performance problems, such as a loss of power, reduce your speed and try to reduce the demands on the engine. For example, it would be a good idea to stop towing a trailer. Have the car checked as soon as possible to prevent expensive damage.
  • Use built-in diagnostic services, if available. Many modern cars have integrated remote diagnostic capabilities, with the ability to report on trouble codes and schedule a service appointment. Today, many automakers—including Fiat Chrysler, Ford, Hyundai, Jaguar/Land Rover, and Volvo—support remote diagnostics and the ability to schedule a service appointment.