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So you come out to start your car one morning and the Check Engine light on the dashboard comes on…and doesn’t go back off again. You can’t really notice any difference in the way the car runs and drives, but it’s on anyway.

What does it mean?

Since the late 80s, most engine functions have been controlled by a central drivetrain computer. This includes emissions controls, fuel metering and delivery, ignition timing, shift points and many other elements of drivability and performance. The drivetrain computer relies on information from a chain of sensors that monitor exhaust composition, camshaft position, throttle position and many other factors.

The voltage readings from any of these sensors are supposed to fall within a certain range. When these readings are out of normal parameters, the drivetrain computer stores a trouble code and illuminates the Check Engine light (also known as a Malfunction Indicator Lamp or MIL). Some problems on some makes may take several failure cycles before the MIL will illuminate.

The trouble code could originate from something as minor as a loose gas cap or a loose or corroded wire to a sensor. On the other hand, it could mean something considerably more serious.

The trouble code can be accessed by a technician using a code reader or scanner device, which hooks up to a standardized connector usually found under the dashboard. The code (or codes) can then be interpreted to diagnose the problem. It takes an experienced technician, however, to know why a given code occurred in the first place and find the problem and repair it.

So for you, the driver, what should you do when the MIL is illuminated?

You should, of course, get your vehicle to a shop for a proper diagnosis as soon as you can. If you notice problems like rough running, poor idle, transmission roughness, smoke or other real driveability problems, don’t put it off! The good news is that the whole system of onboard diagnostics has made it easier for a technician to track down a problem and address it, taking a lot of the guesswork out of the process.