What Does the Check-Engine Light Mean?

CARS.COM — The check engine light on the dashboard is a warning that many vehicle owners dread. The check engine light is a signal that the onboard diagnostics system (or OBD II) has detected a malfunction in the vehicle's emissions, ignition, fuel or exhaust systems. It could be  caused by something as simple as a loose gas cap or a faulty oxygen sensor or spark plug or something as severe as a faulty catalytic converter or major engine problems, so you shouldn't ignore it. All cars and light trucks have onboard diagnostics that are supposed to detect engine-related problems that affect the emissions control systems.

The check engine light (typically a yellow or orange outline of an engine with the word "Check" or "Service Engine") should come on for a few seconds every time you start the engine with other warning lights. If it stays on, that means there is a problem. If your check engine light is flashing or blinking rather than staying illuminated, it indicates a serious problem that needs to be dealt with right away. Many of the common causes of an illuminated check engine light are simple and easy to solve, but sometimes, an engine light means something serious has gone wrong. 

Related: More Electrical Advice

If the check engine light is flashing or blinking, that usually indicates a misfire or other serious issue that requires prompt repair and should be dealt with quickly at an auto repair shop. Ignoring a flashing light increases the chances of additional problems, including damaging an expensive catalytic converter (which costs more than $1,000 to replace on some cars). If your engine light is blinking, take your vehicle to your mechanic right away. 

If the engine light is illuminated but isn't flashing, before rushing to a mechanic, you should first tighten the gas cap because a loose gas cap can trigger a check engine warning. See if the light goes off after several engine starts over the next day or so. Replacing a worn gas cap that doesn't fully seal may also solve the problem.

If that doesn't do the trick, an auto technician will need to diagnose the problem by electronically tapping into an OBD II connector under the dashboard to read diagnostic codes that will help isolate what tripped the check engine warning, such as a bad spark plug, oxygen sensor or another sensor. Many auto part stores also offer free diagnostic testing. While they will not repair your vehicle, they will read the code and let you know what the problem is. If it's a spark plug or an easily accessible sensor, you may be able to pick up what you need and replace it yourself without making a trip to a repair shop, if you prefer the do-it-yourself approach to car repair. 

Even if your vehicle seems to be performing well and your mileage isn't dropping, it's a bad idea to just ignore a check engine light. Something is wrong, and it's likely to get worse. In addition, if you live in an area where vehicles have to pass periodic emissions tests, an activated check engine light usually means your vehicle will automatically fail. Depending on what's causing the light to be illuminated, some problems, like a misfire, bad spark plug, faulty sensor or even bad fuel could decrease fuel economy. 

Cars.com's Editorial department is your source for automotive news and reviews. In line with Cars.com's long-standing ethics policy, editors and reviewers don't accept gifts or free trips from automakers. The Editorial department is independent of Cars.com's advertising, sales and sponsored content departments.