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What Does the Check Engine Light Mean?

DIY, Engines & Components, Guides  /   /  By Nina Russin

Nobody likes looking down at the dashboard and seeing “Check Engine” lit up. It’s designed to alert motorists about a potential engine hazard, but can be a terrible nuisance if there’s no obvious sign of a problem. Does it mean that your car is about to die?

The short answer is: no. The illuminated check-engine light is simply indicating that one of your vehicle’s onboard computers received a trouble code. In many cases, the cause is minor and easy to remedy—such as a bad sensor. But the light can also reveal a more serious mechanical problem so it should not be ignored.

Typically, trouble codes indicate that your vehicle’s emissions system out of compliance. For example, if you neglect to tighten your gas cap properly after refueling, it will trigger the check-engine light. The fix is simple: re-tighten the cap and drive for about 30 minutes, at which point the code will clear, and the light will go off.

Pulling the trouble code with an inexpensive OBD2 reader can quickly put your mind at ease.

Pulling the trouble code with an inexpensive OBD2 reader can quickly put your mind at ease.

A knock sensor with harness-pair kit for Chevy, GMC, or Cadillac

A knock sensor with harness-pair kit for Chevy, GMC, or Cadillac

Any car produced after Jan. 1, 1996 is equipped with OBD2, the second-generation of onboard vehicle diagnostics implemented primarily to help monitor emissions issues. Unlike the similar systems that preceded it, OBD2 is universal. That’s good news for car owners. Using a scan tool or code reader, it’s easy to tap into the vehicle’s diagnostic link and pull the trouble code.

Fixing the problem, however, might not be as simple. That’s because a problem indicated by a sensor out of spec might not be confined to the sensor.

For example, a knock sensor located on the engine block, cylinder head, or intake manifold picks up vibrations caused by pre-ignition fuel in the engine cylinder igniting too early, which can lead to detonation and piston-head damage. Pre-ignition has a variety of causes, ranging from the engine’s base timing being out of adjustment (something the onboard computer does not monitor) to a tank of bad gasoline burning unevenly.

A set of oxygen sensors for Ford vehicles

A set of oxygen sensors for Ford vehicles

Some of the engine’s sensors are difficult to access, and can be even more difficult to replace. The O2 (oxygen) sensor, located in the car’s exhaust system is a perfect example. Depending on the vehicle, the sensor can be located ahead of or behind the catalytic converter. In either case, the location makes the sensor subject to heat and contamination, both of which can cause it to freeze in place. If you don’t know how to remove and replace these sensors, it’s best to leave the job to an expert.

Take Notes

What’s the best course of action if your check-engine light goes on? First, check the gas cap—an easy fix. If this doesn’t solve the problem, you can either pull the code yourself or contact a trusted local service shop, which might perform the service for free.

Also, put a note pad in the car and jot down any changes you notice in the car’s performance and when these changes are most noticeable. Is the ignition harder to crank over? Does the engine seem to lose power during acceleration, or has fuel economy dropped? All of this information will help the technician more quickly get to the root of the problem.

Finally, make sure the person looking at your car is certified by ASE (The National Institute for Automotive Service Excellence), which is a not-for-profit organization that certifies automotive technicians using standardized tests. Even if you can’t fix the check-engine problem yourself, collecting the right information goes a long way in helping to resolve the issue—so the pesky indicator light can once again go dark.

About the Author

Nina Russin is an ASE certified automotive technician and writer who has been covering the automotive industry for 30 years. She was a weekly automotive columnist for the Chicago Sun-Times for 10 years, and a contributor to AutoWeek, Automobile Quarterly, Collectible Automobile, Cycle World, and AAA Arizona Highroads Magazine. Russin is co-founder and president of Active Lifestyle Vehicle of the Year, an annual competition.

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