Aaaarghhh! The Check Engine light just came on!
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You're driving down the road munching on your Big Mac and rocking out to the tunes on your iPod when all of a sudden there's a new feature on your car's instrument panel: the "Check Engine" light just came on. Oops, now what!?

Look at your owner's manual first. Their advice is usually something like "If the vehicle is still running, get it to a mechanic as soon as possible." That's a good idea, but what do you do next?

What Does the Check Engine Light Mean?

First, a little history: the Clean Air Act of 1990 requires that every car or light truck manufactured after 1995 have an on-board diagnostic computer. This computer, commonly called the OBD-II because it's  the second generation of the device, monitors sensors in your vehicle's fuel and emissions-control system. If a sensor reading falls outside its "normal" range, the OBD-II does two things. First, it captures a code that identifies the problem and second, it turns on the Check Engine light.

There are two kinds of codes: a standard set that is the same for all vehicles and a second set that is specific to your make and model of vehicle. All codes can be read with handheld devices called code scanners, which are available at hardware and auto parts stores.

Handheld OBD-II code scanners
Credit: Actron Corp.

How to Read a Code

  1. Get a code reader. You may be able to borrow one from a relative or friend, and many auto parts stores will loan you one. Buying a scanner may also be an alternative, since garages and dealers will usually charge more than the cost of a new scanner just to read the code and tell you what's wrong.
  2. Plug the scanner into the OBD-II port. This is a small plastic outlet that must be within about a foot of the vehicle's steering column, beneath the dashboard. Just look at the plug on the scanner and then search under the dashboard for something that looks like it will fit (see the image below). There may be a plastic dust cover on the OBD-II port.
  3. Read the code from the OBD-II. Follow the directions on the scanner: you may need to turn on the ignition or other steps. Write the code down. Most scanners allow you to reset the computer and erase the code. If you're lucky, the light will stay off when you turn the engine back on.
An OBD-II scanner port
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Research your Code

There are many online sites where you can learn the meaning and cause of an OBD-II code and possible remedies. Sometimes the problem is as simple as tightening the fuel filler cap, sometimes it is more serious. I often use the site OBD-Codes dot com for research because it has links to repair tips, but you can find others with any web search. You can even enter the code directly into a search engine. 


The code P0171 is a  standard code related to the fuel injection system. A sensor has reported that the air-fuel mixture is too lean, causing surplus oxygen in the exhaust. If the sensor is correct, you may experience some knocking, hesitation on acceleration, or a lack of power.

With a little research, you will find that code P0171 can be caused by a dirty Mass Air Flow (MAF) sensor or by a vacuum leak in the air circuit between the engine and the MAF. A dirty fuel filter can also cause this code. Armed with that knowledge, you may be able to repair the problem yourself or get help from a mechanically savvy friend. Best of all, the light may go off and stay off  when you reset the OBD-II computer.  



Cars and trucks built from the late 1980s to the 1994 model year used a different system called OBD-I. Instead of using a computer port, you read an OBD-I diagnostic message as a "blink code." If your vehicle falls in that age range, check the owner's manual for methods of reading and interpreting blink codes, or research OBD-I on-line. Vehicles in the 1994 model year may be equipped with either OBD-I or OBD-II systems, depending on the make and model.