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What Exactly is the Check Engine Light?

What Exactly is the Check Engine Light?

check engine light

In the movies featuring airplanes, the “flashing red warning light” is a thematic indicator that “something has gone wrong” and “the hero must save the day.” Granted, a warning light in the real world doesn’t usually mean your life is at stake, but the check engine light (CEL) can be worrying, particularly if everything else seems to be working fine. What exactly is the check engine light or malfunction indicator lamp (MIL)?

Behind the Check Engine Light

check engine light 101

One only needs to open the hood on a late-model car to see it’s a complicated affair of electronics. The engine control module (ECM) uses these sensors and actuators to get the best performance and fuel economy out of every drop of gas. Software enables the ECM to adjust for most any driving scenario, low speed, high speed, hot days, cool days, even differing grades of fuel. Still, these electronics don’t last forever, parts can fail, and things just go wrong. The check engine light is the ECM’s only way to notify you that, just like in the movies, “ something has gone wrong .”

Sometimes the problem is obvious, and the check engine light illuminates about the same time as you feel the transmission shifting wrong or the feels like it has no power. Other times the problem isn’t so obvious, such as an evaporative emissions (EVAP) problem or wheel speed sensor (WSS) fault. Some problems may only manifest themselves in decreased fuel economy, which few drivers monitor regularly, or even years later as a clogged catalytic converter. Of course, in many States, you can’t pass emissions inspection with an illuminated CEL.

The real problem comes with trying to figure out why the CEL came on in the first place. Maybe you’ve considered getting a tune-up or you’ve tightened your gas cap, but there’s no way to know why the check engine light came on unless you or someone you know has a scan tool and knows how to diagnose it. This brings up a valid question, then: “Who can diagnose my check engine light?” Auto parts stores may offer “free” CEL diagnosis, while your local auto repair shop might charge upwards of $100. Also, you could buy your own scan tool for $30 to $300, but would any of these options get you any closer to a repair? Should you pay for auto diagnostics?

Check Engine Light Hero


As vague as the check engine light is, it doesn’t tell you what the problem is any more than the scan tool does when it reveals one or more of over 10,000 diagnostic trouble codes. Some DTCs are easier to interpret than others, but none of them give a direct answer, like “Replace the Finnigan Valve” or “Don’t Run the Engine When Refueling.” Therein lies the problem with free scanning and even DIY scanning. An “oxygen sensor code” doesn’t mean you should replace the oxygen sensor any more than an online post can diagnose your car from 3,000 miles away. Even statistical analysis can’t guarantee you’re getting the right part or repair for your check engine light problem.

Really, the best an auto parts store can do is “scan for codes,” but cannot take the time to properly diagnose it. After all, they’re auto parts stores, not auto repair shops. Even if you have your own scan tool, you might be limited to “scanning for codes,” unless you know your way around a wiring diagram and digital multimeter. Unless the damage is obvious, your best bet is a check engine light hero , your local auto diagnostics master auto repair technician.

It shouldn’t come as a shock that check engine light heroes are paid, because they’ve put in thousands of hours of their lives to learn everything there is to know about their cars and trucks, auto repair, and auto diagnostics. Many of them specialize in certain brands, which makes their expertise even more valuable when it comes to diagnosing what’s behind your check engine light. Finally, aside from training and experience, auto diagnostic professionals have access to far more powerful tools than the average DIYer or auto parts store, which they may have purchased themselves. Only professional auto diagnostics technicians have the training, experience, dedication, and tools to properly diagnose your check engine light. Anything less is just guessing, and guessing is a waste of time and money.