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Buying a Used Car: What Does the Check Engine Light Mean?

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author photo by Doug DeMuro April 2014

If you're thinking about buying a used car, you have to assume that some of the cars you're considering won't be perfect. In some cases, you might even discover a check engine light that's on during a test drive. Is that a big deal? We have some answers about what a check engine light means and whether it's worth worrying about.

What Does a Check Engine Light Mean?

Unfortunately, there's no easy answer to this question. At its core, a check engine light turns on when the car senses that there's a fault somewhere in the engine, typically with a system that relates to vehicle emissions. But the exact fault isn't spelled out in the gauge cluster. That's by design, considering the sheer number of potential faults that a car engine can have.

With that said, the most common check-engine-light problem relates to a loose gas cap. You can lose a lot of gasoline out of a loose cap, so the light will turn on to inform you that the cap is loose. If you see a check engine light, and the car is running fine, first try to tighten the gas cap. If the light still shows up after a few more short drives, it's probably something more serious.

Have a Mechanic Read the Code

Every time a check engine light turns on, it includes a code that's stored in the vehicle. A mechanic can read this code using a special workshop tool. Given the potential that a check engine light might be an expensive fix, we strongly recommend visiting a mechanic if the car you're considering is showing a check engine light.

While many mechanics charge a diagnostic fee to read a check engine light, it's usually worth every penny, as it could save you far more money down the line. Consider an example: If you pay $50 to have the check engine light examined and you discover that fixing the problem will cost $1,000 or more, you've saved yourself a lot of money, even if it feels like you're out $50.

Don't Assume That It's Nothing

Maybe the most important rule of thumb for when you consider buying a used car with the check engine light on is that you should never assume that there's no problem. The light goes on for a reason, and even if the seller says something about it being broken, we strongly suggest having it checked out by a competent mechanic. At the very least, this will give you full knowledge of the issues and problems with the car that you might buy.

Don't Walk Away From the Car

It's also important to remember that a check engine light doesn't always indicate serious problems. If a mechanic checks the light and you learn that the car needs a $1,000 repair, consider asking the seller to drop the price by $1,000.

This is especially important to remember in situations where you're considering a rare car or one that has an unusual color combination that you happen to like. Since you're unlikely to find another car that fits your needs, it's usually a better idea to try to negotiate a fair price on the car that you're already considering instead of restarting your search for a whole new vehicle.

This image is a stock photo and is not an exact representation of any vehicle offered for sale. Advertised vehicles of this model may have styling, trim levels, colors and optional equipment that differ from the stock photo.
Buying a Used Car: What Does the Check Engine Light Mean? - Autotrader