PhotoIt can be disconcerting to be driving along and suddenly have your vehicle's "check engine" light come on. The light is meant to get your attention, but keep in mind it can be a warning of something simple or something more serious.

For Tim, of Parker, Colo., when the check engine light came on in his 2005 Nissan Pathfinder, it was something serious.

"I drove it in to the dealership and reported that for about a week the transmission was slipping," Tim wrote in a ConsumerAffairs post. "The service rep, when he heard it was transmission related, asked me right away, 'does it stutter at about 40 mph when shifting?' I said it did and he replied, 'that's not good.'"

Decimbra, of Jackson, Miss., says the check engine light in her 2006 PT Cruiser was an indicator of something less serious. At least, that's what she was told.

Just needs a tune-up

"I was told that my car needed a tune up so Firestone kept my vehicle from 7:04 am until 1:01 pm to complete the tune up," she wrote at ConsumerAffairs. "When I picked my vehicle up I was told that the light would not come on and that everything was done that needed to be done."

Decimbra says a couple of weeks later the light was back on again.

"On my way to work my car began to pull and make a strange noise," Decimbra reports.

Since the 1980s vehicles in the U.S. have had onboard diagnostic systems, tied into the car's computer. When the computer senses a problem, the check engine light comes on. The only problem is, it only tells you should check you engine, it doesn't tell you why.

In many cases, the light signals a problem with your car's emission control system; the "O2" sensor is a frequent culprit.  It's often a simple fix but it's one you shouldn't put off since it affects your fuel economy as well as what's coming out of your tailpipe.  If you live in a state or region where cars must pass an emission test every year or so, you can be certain your car won't pass if the light is on, so it's worth getting it checked out.

Code reader

A relatively inexpensive device can tell you why, and if you have a persistent problem with your check engine light and are getting little help from your auto repair shop, you might consider investing in a check engine code reader. Many of these small devices sell for around $60.

These code readers are designed to work on 1996 and later models, which have the On Board Diagnostic (OBD) II system. The devices come with an attached 16-pin connector cable that connects to the output of the diagnostic system, located under the dashboard. The device will register the "trouble codes" that are triggering the check engine light. It also allows you to turn off the light.

That will tell you right away how serious the problem is. When you take the car in for repair, you can tell the service personnel what the problem is. Whether you trust your mechanic to find and repair the problem or use a code reader to do it yourself, automotive experts say the one thing you should not do is ignore the check engine light.

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