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.
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 power train.” 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. 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.
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:
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.