Check Engine Light Flashing: Why It Happens & What to Do About It

Vehicles have become very advanced over the past three decades, and they can do almost everything nowadays – from make on-the-spot adjustments in fuel delivery and throttle inputs, to drive nearly completely hands-free.

It’s very exciting to think about where we’ll be in 10, 20 or 30 years from now!

Nowadays, our vehicles also let us know when there’s a problem with one or more of the systems that make them run.

All sorts of warning lights illuminate when an issue is detected by the vehicle’s computers, but no light is more ominous than the check engine light.

Car dashboard lights illuminating to warn of possible engine problems

It’s bad enough when that light comes on, but it’s even worse when it starts to flash. This is a sign that there is a serious problem occurring, one that shouldn’t be ignored under any circumstances.

If the check engine light starts flashing in your vehicle, get it to the nearest mechanic shop as soon as you can so they can diagnose and fix the issue immediately.

Until then, and to get you prepared for all of this if you ever see the check engine light pop up in your dashboard, here’s everything you need to know about it.

What Is the Check Engine Light?

The check engine light is a small icon with a vehicle’s gauge cluster which illuminates if the vehicle’s ECU, or “Electronic Control Unit”, receives a fault code from one of its sensors, actuators, or systems.

This is a function of the On-Board Diagnostics system.

Starting in 1996, the EPA mandated that every vehicle sold should have a standardized system with which to monitor computer systems in vehicles. This is called On-Board Diagnostics II or OBDII.

An OBD II stores each error code received by the ECU, and can be read by an OBD2 scan tool to show exactly what each fault code represents. After all of this is done, a diagnosis can be made.

Even if it is not currently illuminated in your car’s dashboard, an OBD II scanner has the ability to see previous fault codes sent to the ECU, and is the answer for  how to reset the check engine light if you so desire.

What Would Cause the Check Engine Light to Come On?

The reason your it comes on is because of a non-adjustable event that occurs with a vehicle’s computer systems.

Because modern vehicles are so technologically advanced, at least that’s the case when compared to their older counterparts, their computer systems are usually able to adjust themselves to changing conditions.

A good example of this is the fuel system increasing or decreasing the amount of fuel delivery to the engine depending on how hard you press the accelerator.

ECUs can even fix small problems by compensating with different factors.

It will come on when the computer detects an irregular condition and isn’t able to bypass or compensate for the issue.

A flashing check engine light is usually a sign of a more serious than usual problem the vehicle is experiencing. Usually, it means that several systems are failing, or one major system is being heavily compromised.

A blinking check engine light is a worst-case-scenario situation, and shouldn’t be confused with intermittent illumination.

Intermittent illumination occurs only when an issue is being actively detected, and could go off for several days at a time and then re-illuminate for several more days.

If it stays on, this is not something you should overlook, but is not necessarily something that signifies immediate danger or severe damage.

What Would Cause the Check Engine Light to Flash?

It will not usually flash, unless there’s a major problem occurring with the vehicle’s fuel or emission system.

The most likely issue that would cause flashing is when the catalytic converter is damaged because of the problem that’s occurring.

The following list discusses some of the most common problems that might cause this light in your dashboard to flash:

The Engine Has a Misfire

A misfire occurs when fuel is injected into the combustion chamber, the piston moves up to compress the air and gasoline mixture, but no combustion occurs.

This could happen for multiple reasons such as a fouled spark plug, a faulty injector or injector system, or even piston damage.

It’s the most common reason the check engine light starts flashing, as undetonated fuel could leak through the exhaust system and make its way into the catalytic converter.

Unburned fuel which reaches the catalytic converter could actually burn inside the converter itself and melt its interior structure, causing obstruction of exhaust flow.

This could also prevent the harmful exhaust gases from catalyzing, thus ruining the catalytic converter.

If you need a fix for this, know that the average catalytic converter replacement cost is around $1400.

There Is an Ignition System Defect

The check engine light could start flashing if the spark plugs, coils, or seals within the ignition system have failed or are leaking. This is one of the more common reasons a misfire could occur.

Oil can leak into the combustion chamber and prevent the spark needed for detonation, causing a misfire. Unburned oil can also have the same effect as unburned fuel, if it passes through the exhaust manifold and into the catalytic converter.

Changing spark plugs or O-ring seals are both common maintenance procedures which you can do at home to see if it solves the issue.

Click here for details on how often you should change spark plugs.

Engine Damage or an Engine Component Defect Has Occurred

Even if the catalytic converter may not be directly affected by engine damage, it could also cause the check engine light to start flashing.

Timing chain, valve, and piston damage are rare causes for this, but they are among the most serious reasons this happens.

Most likely, there will be other signs of engine damage that appear before or while any warning lights would illuminate, including loud engine noises, a shaking vehicle, or even engine cutoff.

Any engine component damage could cause the entire engine to fail, requiring a replacement.

There Is a Fuel System Defect

Any time a fuel system component fails, a sensor is sending wrong information to the ECU, or another system sensor is receiving faulty information, there is a possibility that your check engine light could start flashing.

Fuel system defects are a primary cause of an engine misfire. Fuel is delivered to the engine in varying amounts depending on factors such as throttle position and air intake. If any of these sensors were to fail, this could cause more or less fuel than is actually needed to reach the engine.

Fuel Leak

A fuel leak is another culprit. Fuel could be pumped to the engine, but never received by the engine. Fuel could also be leaking around or inside the engine itself.

Faulty Oxygen Sensor

A faulty oxygen sensor could fail to deliver the correct amount of fuel to the engine.

Even a working oxygen sensor could send what it believes to be the correct amount of fuel to the engine, because an air flow sensor is providing incorrect readings.

Faulty EVAP System and EGR Valve

Both the EVAP system and EGR valve could be faulty.

EVAP stands for Evaporative Emissions Control, and is a system that neutralizes gasoline fumes emitted by a vehicle’s gas tank.

EGR stands for Exhaust Gas Recirculation, and is designed to recirculate exhaust gasses to a vehicle’s intake in order to mitigate harmful emissions.

When either of these systems fail, a vehicle’s emissions system is compromised and could cause the check engine light to flash.

A common and surprisingly easy way to fix this problem is to unscrew the gas cap and screw it back on. This does not always fix the issue, but many times – you’ll be surprised that it will.

There Is a Vacuum Leak or Gasket Failure

Anytime air, oil, or gasoline bypasses a sensor and leaks into places it shouldn’t, your engine and other components might be at risk for failure.

A vacuum leak could cause additional air to seemingly enter the engine, causing the ECU to overcompensate and send additional fuel when it isn’t needed.

This again could cause a misfire due to excess amounts of fuel within the combustion chamber.

Likewise, a gasket failure could cause coolant and oil to leak into the engine and eventually the exhaust, possibly resulting in catalytic converter failure.

There Is an Emission System Fault

An engine misfire is not the only cause of catalytic converter failure and emissions system problems.

You see, catalytic converters can wear out with age. Even though a catalytic converter should last for a long time, they still wear out and become clogged over time.

An overheating engine is another common cause of catalytic converter failure, and can be compounded by an oxygen sensor failure.

You Have a Faulty ECU or Other Sensors

ECU failure is rare, but can occur. If it does, there will probably be a host of other problems that accompany it at the same time.

Apart from the ECU, there are anywhere between 60 and 200 different sensors working to keep your vehicle performing properly.

Not every single one of these is vitally important to emissions, fuel delivery, or engine function, but one (or several) that fail could cause the ECU to illuminate the check engine light or make it flash.

In these situations, using an OBD II scanner will tell you which sensor is malfunctioning.

Your Gas Cap Is Loose

Consider yourself very lucky if you find yourself in a situation where this happens because of a loose gas cap.

Most vehicles have a separate gas cap light that can also flash if there is a major system malfunction, but simply tightening it or screwing it back on can eliminate the problem.

Again, this sounds ridiculously simple (and it really is), but don’t laugh it off! You’d be surprised at how many times this turned out to be the culprit.

You Have a Broken Engine Mount

Another scenario you’ll be lucky to go through that would cause your vehicle’s check engine light to flash is a broken engine mount.

The engine has to be attached to the vehicle, and engine mounts can wear out and snap off – whether by wear and tear or because your vehicle has been well used.

This could cause severe shaking, which – in turn – could result in seals, other bolts, and wiring across the engine.

If you experience severe shaking in your car and your this light starts flashing, stop and turn off the vehicle (as soon as you get the opportunity to do so somewhere safe) to prevent damage from occurring.

There Is an Electrical Short or Other Electrical Issue

Wiring and electrical issues can cause a whole host of problems for a vehicle, and the lack of electricity could do the same.

Anytime a sensor stops receiving information, it can no longer transmit information to the ECU and will cause a fault code.

If the ECU can’t receive the data it needs, it could do anything from make the check engine light flash to shut off the vehicle, depending on which sensor isn’t receiving any information.

A disconnected wire or lack of battery or alternator power could also cause this to happen, but is unlikely to do so.

For more useful information that could come in very handy when you’re trying to get to the bottom of this, you can watch this video.


What Should You Do If the Check Engine Light Starts Flashing?

If this starts happening while you are driving, you should take the vehicle to the closest mechanic shop to have them look at the fault codes.

If the vehicle starts behaving erratically, find the closest pull-off area so you can safely park and turn it off. The same should be done if there are unusual noises or if the vehicle starts shaking.

If it starts blinking while you are parked, you can still opt to drive it to the nearest repair shop, but towing it is a safer option.

Never ignore it when it’s flashing, as this is usually an obvious sign of a serious problem.

If absolutely necessary, you should be able to drive the vehicle for a very short distance if it still runs, until you get somewhere you can safely turn it off.

Under no circumstances whatsoever should you drive the vehicle for extended periods of time or long distances with the check engine light flashing. This is very important, both for your safety and your vehicle’s well-being!

Take It to a Mechanic

Because a flashing check engine light is often the sign of a serious issue, you often won’t be able to fix the problem yourself.

Even if you have your own OBD II scan tool that you know how to use, mechanics usually have much more expensive versions that pinpoint issues more accurately. This is their profession and what they do for a living, after all.

The Sooner, the Better

Catalytic converter damage, engine component damage, and sensor issues often require replacement rather than simple repairs.

The sooner you diagnose the issue and the less time you drive your vehicle if the check engine light starts flashing, the greater the chance that a major issue will be avoided.

Don’t Panic

Never panic when you notice this is happening, especially if you’re driving. Panicking while in traffic is one of the worst things you can do for yourself and others around you.

Instead, simply find the best and safest place to pull off the road. If you need to do so, call a trusted friend for help.

It’s Not Always a Huge Problem

Don’t always assume the worst – because oftentimes, it could be a very, very simple issue going on.

The On-board Diagnostics system in your vehicle is designed to alert you to problems in order to prevent something worse from happening.

One of the more common reasons this happens is because the gas cap is loose, and – as we’ve discussed in a section above – fixing this is ridiculously easy and straightforward.

Always do whatever you can to check anything that might be “too obvious.”

You may not be able to tear your engine apart on the side of the road, but you can inspect the engine, check for any leaks, and secure any wiring, hoses, or filler caps to see if that helps.

If it does, then congratulations! You’ve just saved yourself a whole bunch of time and – very possibly – a whole bunch of money.

Wrapping it Up

There are so many moving parts that go into a vehicle, and so many computer systems that work together to keep them running properly nowadays.

Most of them intertwine with one another and have a domino effect if just one fails.

Rarely is there ever only one issue that will cause your check engine light to flash. Sometimes, the codes received by the ECU don’t even reach the main source of the problem, but are side effects of a larger issue.

Regardless of what exactly is happening to your vehicle, you should never ignore this event.

Ignoring it could cost you thousands of dollars in damage, and a much bigger problem on your plate than what you already had.

Instead, doing whatever it is you can do to inspect your vehicle can solve the entire issue – oftentimes with just a simple twist of a cap or tightening of a screw.

Whatever you do, though, always remember never to panic! And always keep in mind that a mechanic is always standing by for help if you need it.

I've had a passion for cars since 8 years old, and been a subscriber to Auto Week magazine since my 10th birthday. Ever since I turned old enough to drive, I have driven as many vehicles as possible, while teaching myself how to perform maintenance and upgrade work on every vehicle I've owned. For the past 10 years, I've been honing my skills as a vehicle hobbyist, in recent years also enjoying writing car reviews, opinion articles, vehicle how-tos, car-buying guides, and even provide individual consultations for those who need car-buying advice. In addition to writing for Vehicle Scene, I currently write for Autolist, and also own and operate my own vehicle blog website, The Unlimited Driver.

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