The most common causes of check engine lights

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4 min read

The check engine light (or CEL) is a friendly way for your car to say: “Chickity-check yo' engine before you wreck yo' engine.”

Less friendly ways include breaking down on the interstate (so don’t freak out just yet!), but when you notice your check engine light illuminated on the dash it does mean that you’re going to have to take a pause to figure out what the problem is.

These lights are part of your car's onboard diagnostic system. It sounds hi-tech but computers have been controlling and monitoring vehicle performance for decades—and when this system picks up on something that it can’t fix itself, it flicks on the check engine light.

The issue can range from something relatively minor, which requires some basic car maintenance, to an expensive fault. If it’s the latter, it may be a sign that it’s time to sell your car to Peddle, but whatever the cause, it’s good to be clued up on the most likely scenarios.

5 common causes for your check engine light to come on

a loose gas cap

If you’re lucky, the issue causing the CEL to turn on will be a loose gas cap (which is actually pretty important for keeping the engine pressure, and preventing excess fumes). It’s also a really easy component to check, right? Give it a quick tighten (and test that it’s screwing in correctly) and it may well solve the problem.

faulty catalytic converter

The catalytic converter is a very important (and very valuable) part of your car’s exhaust system. It converts carbon monoxide into carbon dioxide. But it can get clogged, particularly if you drive shorter distances. Signs your catalytic converter is running into trouble include smoke and noise coming from your exhaust.

faulty air flow sensor

Your air flow sensor, also known as the mass airflow sensor (MAS) measures the air entering the engine in order to maintain the correct air to fuel ratio. It can also get dirty and clogged up, so if your check engine light is on, it could be worth checking if the MAS needs a clean, or to be replaced altogether.

old spark plugs

Worn spark plugs are another common problem that can cause your CEL to illuminate. If the plugs are worn down—or the ignition coil, which provides the electricity for the “spark”—it can cause the engine to misfire. Luckily, spark plugs are cheap and easy to replace. So consider this one a minor.

broken oxygen sensor

While the MAS measures the airflow in your engine, the oxygen sensor keeps track of the unburnt oxygen in your car exhaust. Problems with your oxygen sensor can have a knock on effect on other components, like the spark plugs, and mean you end up burning through more gas than you need. It may need replacing.

is it safe to drive with a check engine light on?

If the light is flashing (in some cars it might turn red), that points to a serious problem and you should pull over and call a mechanic. If it is steady, then the issue probably is not urgent (but still needs attention), and you are safe to drive for the time being.

what should I do next?

Your onboard diagnostic system stores a diagnostic code whenever it registers a fault. This will help whoever is fixing the problem figure out where to start looking.

To find out the code, you need to use a fix finder tool (some of the common codes are listed here). You can buy one of these tools yourself, or swing by your local auto-store where you can often get it checked for free. Once you’ve got a handle on the underlying problem, you’ll need to price out the parts and labor.

If it’s looking like a repair will come to a hefty bill, it may be time to trade in your car for a newer one. Here at Peddle we love every car–even if the CEL is flashing. Sell it to us and light up your life with some extra moolah.