Thurston Car Talk: My Check Engine Light Is On


Submitted by Bron of Bron’s Automotive

A question I get asked frequently is “My check engine light is on. What does this mean?”  Good question.  We are talking about the amber light on the dash that reads check engine, service engine soon, service engine now, or we sometimes see a little picture of an engine that is amber in color. These all mean the same thing.  The simple answer is that it means that the vehicle’s on board computer has detected a problem with a sensor or a system that it routinely monitors, set a trouble code that indicates what problem or system the computer has detected problems in, and turned on the light on the dash to alert the driver there is a problem that needs to be repaired.

Since passage of the federal clean air act, and starting with the 1996 model year, all car makers are required to meet certain minimum criteria in this area, in accordance with the law.  On board computers in autos are constantly testing themselves and the vehicles wiring and sensors and certain other systems.  If your check engine light was on for a while, but then went off, it means that the system or sensor in question initially failed, but then later passed two retests.  The computer clears the code and turns off the light by itself.   A good example of this is if you leave the gas cap loose.  These tests are only run when the fuel tank is between ¼ and ¾ full, so if your check engine light turns on at ¾ tank, check the fuel cap.  If it was loose and you retighten it, simply keep on driving normally.  After a day or so, the tests will run a few more times and pass, and the light will turn off by itself.

More critical systems that are being monitored, such as fuel trim, or oxygen or throttle position sensors, will turn the light on and keep it on until you take the car to the shop to have it diagnosed by a competent technician.  Turning the car off and then restarting will not turn the light off in this case.  As a general rule, it is safe to drive it in to the shop, after first checking oil level and automatic transmission fluid level.

When the computer detects an engine condition that may damage the catalytic converter, (an expensive part), it causes the check engine light to flash. Never drive with a flashing check engine light.  Pull over and get out that cell phone and call AAA for a tow.  Failure to do so may result in a higher repair bill.

The common theme in these three ways that the check engine light can come on is that it always means that there is a defect that is increasing emissions from your car, whether from increased gasoline vapors, or increased emissions from an engine that is not running right.  Some of these defects have the potential to harm the engine if not attended to.   Some are tempted to continue driving for a long time with the light on if they feel the car runs OK, but this is not a good idea.  If the computer later detects a second more serious problem that has the potential to hurt the engine, IT CAN’T TURN THE LIGHT ON A SECOND TIME!  Hope this has been helpful, and remember here at Bron’s Automotive “we care for your car like we care for our own.”

That’s all for now.  If you have a question you would like answered, send an e mail to

About The Author

Bron Lindgren has been an Olympia resident since 1975.   He has been working on vehicles as a technician since the late 1970’s, and started his own shop, Bron’s Automotive in 2002.  Bron hires only ASE certified technicians, is an AAA approved auto repair shop, and provides shuttle service as well as a 2 year/24,000 warranty. Bron is a member of the West Olympia Rotary, the Automotive Service Association, Thurston County Chamber, and a board member of the Community Mental Health Foundation, BHR’S fundraising arm.

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