Most Common Causes of the Check Engine Light - Car Talk (2024)

• Written ByJohn Goreham
• 4/16/2024

If the dreaded Check Engine Light (CEL) has just been illuminated on your dash, resist the urge to reach for your black electrical tape to cover it up. Most check engine warnings don’t result in expensive repairs. In fact, the most common problem causing a check engine light can be permanently fixed for under $20. Be an optimist. You’ll live a longer, happier life.

Okay, if you really want to be a pessimist, the CEL may also mean that your timing belt has broken, the pistons have struck and bent all the valves, and a connecting rod has just shot out the side of the engine block and passed through your left fender to strike the Starbucks cup in a pedestrian’s hand.

Kidding aside, there are 97,351 reasons that a check engine light can come on. Try and catch your breath. We will list the top reasons why a check engine light has spoiled your mood and offer insight into how you and your mechanic can resolve the issue.

CEL Blinking or Flashing? Warning! Take Note!

If your check engine light is flashing, blinking, or otherwise trying its best to get you to look at it, pay attention. Multiple members of the Car Talk Community asked Car Talk to highlight the importance of a check engine light that is blinking at you.

Car Talk Community user Texases had this advice to share with us.

“If the CEL is flashing or blinking, that means you could be damaging the engine by running it. Get it looked at ASAP, avoid driving it if possible.”

“If CEL is on, but also flashing, that’s a sign of impending doom, serious engine damage possible, and the engine should be shut off immediately, and the car towed to shop pronto.”

Should I Ignore the Check Engine Light?

Driving with a check engine light on is always a bad idea. Sure, you can make it home and then to the shop. That should be the extent of your time driving with the CEL glaring menacingly at you. In many cases, the CEL is a good thing, in a way. It is an alert that bigger problems are headed your way very shortly if you don't take action.

Another reason many Americans can’t ignore the check engine light is that you cannot get an annual inspection sticker with this light still illuminated and recorded by the car. If you live in a ZEV state like California or Massachusetts, the car will confess the problem when you try to get your sticker. A car with stored codes in such states is an automatic fail.

Can a Mechanic Just Shut Off This Annoying Check Engine Light?

Absolutely. Any mechanic can make a check engine light go out. For a minute. Until the car’s diagnostic systems, which are more elaborate than Appollo 13s, sense the problem and turn it back on. Sometimes that can take a day, a week, or even a month. But like a toothache, it only gets worse the longer you ignore it.

How Will My Mechanic Figure Out What the Check Engine Light Means?

All modern automobiles have an onboard diagnostics port up under the left side of your dashboard. It is a multi-pin sort of thing-ie into which your mechanic can plug a code reading tool or even a laptop. Once the reader and the OBD-II (oh-bee-dee-two) port, as it's called, synch up, the car spills its guts and starts telling the reader all of its problems in a secret code originally designed by automakers to preclude independent mechanics from being able to fix your car. Hey, support the right-to-repair bill if your state has one!

Check Engine Light Codes

The problems your car already knows about are not sent in the King’s English to the reader but rather in a code. Literally a code. The codes are alphanumeric and often start with P, as in yousagotta Problem.

The code reader your mechanic has purchased for about a bazillion dollars then translates the code into a (way too short) description of the problem. The code does not provide a detailed listing of the problem. However, automakers get this information from internal technical service bulletins. And experienced mechanics know what the codes point to and can then make a further diagnosis.

How to Read a Check Engine Light Code

Check engine light codes are so frequent that many auto shops will read the basic codes for you in a jiffy. Alternatively, some auto parts retailers will help you do it, or they will loan you a scanning tool to do it yourself.

For about the price of a large mushroom pizza, you can even buy a cheapo scanning tool at Walmart, Amazon, or at your favorite brick-and-mortar retailer. For about a C-note ($100 American dollars), you can get a cooler one. It will plug in and then send you the codes via Bluetooth to your smartphone. Onto which you can download an app that will help you get past “P0440” to more details on what the codes might mean. P0440 means check gas cap, in case you are wondering.

Most Common Causes of the Check Engine Light

• Loose gas cap or a faulty gas cap
• O2 sensor issues
• EGR issues
• MAF sensor issues
• Emissions management issues (Catalytic converters)
• Spark-related and charging issues
• Fuel delivery-related issues
• Another problem described by another warning light, but the CEL felt left out
• It’s actually serious

Loose Gas Cap and Check Engine Lights

The great news is that most check engine lights don’t actually mean “engine,” but rather, they mean, “Hey, the gas cap isn’t holding a vacuum.” Technically, this is a Check Gas Cap light. A warning like this can often be resolved by tightening the gas cap. However, sometimes the little spring and rubber o-ring inside the cap are worn, or the cap is cracked and needs to be replaced. An OEM gas cap for a common vehicle like the RAV4 costs about $25. Less expensive ones are available.

Your mechanic can help you clear the codes, or if you bought that reader we mentioned, it might get the job done. Fun fact, your car could not care less if you have a gas cap. It runs fine without one. This warning is one of the many emissions control regulations put in place by politicians who live in a small moist city on the Potomac and hate cars. We have found a marked decrease in gas cap failures on electric vehicles, so they must have improved the design or something.

O2, EGR, and MAF Check Engine Lights

There are a number of sensors that enable modern cars to run properly. Included are oxygen sensors, exhaust gas recirculation valve sensors, and mass airflow sensors. All of these are sniffing your goezintah and goezoutah gasses as you drive along. If they don’t pass the sniff test for any reason, or if the sensor itself has given up the ghost, the car gets a code pointing to this problem. Your mechanic will know what to do, and most of these are not that expensive to deal with.

Emissions Management Check Engine Light Codes

So far, all of the check engine lights we have discussed are related to emissions. The gas cap, the gas sniffers, and many more like them. There are more expensive problems that a CEL may point to related to the emissions system. And the worst among them is the catalytic converter.

Had we written this story three years ago, we would have just said, “it’s likely shot.” However, lowlifes stealing catalytic converters is big business in America today. We hope this never happens to you, but if your CEL is accompanied by your Prius suddenly sounding like a 1967 Shelby Cobra when you start it, you may want to peek under the car and see if there is a lonely space where your catalytic converter used to be.

Spark, Charge, and Misfire Codes

In addition to watching your car’s gasses flow in and out, your engine has a wide array of sensors associated with the electrical systems and the combustion process. Engine codes can report misfires and other electrical issues. These can, in turn, lead to a mechanic checking spark plugs, wiring, and your starter and alternator.

Your author recently had a Subaru Forester intermittently displaying a check engine light. When a mechanic checked it, he could not discern the underlying problem. The code would self-resolve and go out, and later return. To this independent, experienced mechanic who read the code with a pricey reader, the message was vague. However, when the car was taken to a Subaru dealer, the code was immediately identified as the alternator due to an internal Subaru technical service bulletin.

Fuel Delivery Check Engine Lights

Fuel delivery is carefully managed by your vehicle’s computer. If it finds that a fuel delivery issue may be occurring, a check engine light will illuminate, and the code may point to a fuel pump issue.

Check Engine Lights That Illuminate Along With Other Warning Lights

Your vehicle likely has a dozen or more individual warning lights that it can display that have certain meanings. For example, the high-temperature warning light, low oil warning light, and many others. It is not uncommon for a Check Engine Light to also illuminate when these are showing.

Similarly, a CEL, along with another light, such as the low tire pressure warning, could mean something specific, such as “check the ABS system.” Your manual may be helpful.

Check Engine Light - Bad News; It is Actually The Engine

If you need a deep thought to carry you down the highway, ask yourself, “What is an engine?” Well, the block, pistons, connecting rods, camshafts, valves, and connecting rods are all part of the mechanical masterpiece, with tiny bits moving at insane speeds under the hood. Rarely, but every so often, the check engine light means that you actually have a problem with part of the engine. There are times when something dire has happened. A broken timing belt. A cracked block. These are the lightning strike-like events that almost never happen, but if they do, the CEL will be there. Lit.

We are sorry to have ended on a down note, but the bad news had to be in the story someplace. Most often, a check engine light is not a catastrophe. The most common problem signaled by a check engine light is simply a gas cap that needs a turn. If your check engine light should come on, don’t panic. Get it checked out, and the chances are it isn’t as bad as it may seem from the little orange image.

Read more on the topic of Owning a Car here.