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Why Is My Check Engine Light Flashing?

By Joe Ballard
Published on February 9th, 2024

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I have a love-hate relationship with my car. I also have a feeling that I am not alone in this sentiment. What I love about my car (Hyundai Ioniq Hybrid) is the same as what I hate about my car: the complexity. I love all the high-tech systems and gadgets that make the vehicle safe and deliver a gratifying driving experience.

But then there is the flip side: what happens if one of the systems in the car has a problem and triggers the check engine light (CEL) to come on? For instance, issues like a loose gas cap, a bad oxygen sensor, or a failing catalytic converter can trigger the CEL. Now, the complexity that got me hooked into loving the car has the potential to lead me down a road of expensive repairs, but maybe not.

This article aims to help you understand what problems will cause a CEL to come on and what actions you can take to resolve these problems so that you continue to love your car.

why is my check engine light flashing

How To Start Diagnosing Your Check Engine Light

Every vehicle built after 1996 for the US market has a check engine light (CEL), also called a malfunction indicator lamp (MIL), built into the dashboard. The indicator is sometimes shaped like an engine, but it can also be a written warning like “service soon” or “check engine.” The vehicle's make and model control the actual display. Obviously, a CEL only provides a warning that there is a problem, not what the problem itself is.

Pinpointing the issue requires a sophisticated tool called a Scan Tool or OBD2 Scanner designed to read codes stored in the electronic control unit (ECU).

innova 5610 OBD2 scanner for check engine light issues

An excellent example of a Scan Tool is the model 5610 from Innova. Many systems control and monitor the vehicle's performance, including but not limited to engine speed, ignition timing, fuel control, braking systems, transmission function and some even control all these functions to facilitate semi-autonomous Self Driving Applications.

Typically, a code generated by the engine management system (EMS) and stored in the ECU will point you toward the problem. However, narrowing it down further will depend on the level of sophistication of the Scan Tool.

So, if the CEL does come on in your dashboard, it may not mean that you must take immediate action. For instance, you may need to either bring it into a shop for repair or, if you are a DIYer, use your Scan Tool to fix it yourself.

Does It Require Immediate Fix?

The severity of the issue that caused the CEL to come on in the first place varies from something as simple as the most basic emissions control system fault (a loose gas cap), to something more severe, like engine failure.

Whether you decide to bring it into a shop for diagnosis and repair or if you are a DIYer able to tackle it yourself, it is essential that you resolve a CEL fault as soon as possible to avoid doing more damage or, worse, breaking down on the side of the road. So, let's look at what the CEL is telling you about your vehicle.

In most situations, the CEL will come on steady and stay on until the problem is fixed, but in some situations, the CEL will come on and flash. Each situation will indicate a specific level of severity related to your vehicle.

understand why check engine light flashes

A solid CEL is a warning that there is a problem, but it may not require immediate attention unless it is coupled with poor performance or excessive fuel consumption, so getting it checked out as soon as possible is a good idea.

The other situation is when the CEL is flashing. A flashing CEL indicates a more severe problem, such as an ignition malfunction or engine performance issue that could damage the catalytic converter. A flashing CEL is something to take very seriously, as it could become a catastrophic problem.

If you are driving with a flashing CEL, paying close attention to your vehicle’s performance is important. If it is rocking and rolling, emitting a foul smell (like sulfur), or acting strangely, it would be wise to pull over and perform an inspection yourself or call a roadside service (like AAA) to come and help you out.

Find the perfect scanner in 1 minute

Key Takeaways

So, what should you do if the CEL comes on?

It will depend on whether it is on steady or flashing, but in either situation, you don’t need to panic; however, you should also assume there is some problem and take action as soon as possible.

The only way to determine how severe the problem is will involve using an OBD2 scanner. Most parts stores have a scanner and will perform this test on your vehicle for free, hoping that you will buy your replacement parts from them. You can also take your vehicle to a repair shop or the dealership or do it yourself if you have a Scan Tool.

The problem could be as simple as a loose gas cap or as bad as a catastrophic engine failure. Still, you won't know how serious the problem is until your vehicle has been connected to a Scan Tool via the diagnostic port. Doing this will enable you or a professional to retrieve stored diagnostic trouble codes (DTCs) from the computer and then research to find an explanation of the failing component(s).

using an obd2 scanner to diagnose check engine light

The DTCs stored by the ECU use a standard code language to indicate the system where the problem occurs. The DTC consists of a letter followed by a 4 or 6-digit code. Be aware that there are potentially thousands of codes, but they will usually fall into the following groups:

  • P means there is a problem in the powertrain
  • B means there is a problem in the body or interior components
  • C means there is a problem in the chassis or suspension
  • U means there is a problem in the internal network

Diagnosing and repairing a problem affecting your vehicle depends on identifying and understanding the DTC generated by the ECU. Today's vehicles were designed to utilize sophisticated diagnostic tools for maintenance and repair.

The bottom line is that there is no way to diagnose and fix a problem without using a Scan Tool.


There is no need to fear a CEL that suddenly comes on, either steady or flashing. You need to look at this as a blessing in disguise because if you think about it, your vehicle enables you to address a problem before it becomes much more severe.

With the CEL indication as the first line of defense, you could avoid getting stranded or causing more significant damage to your vehicle leading to costly repairs.

Innova's mission is to make diagnostics easier for DIYers and mechanics alike. If you have questions, we encourage you to visit our community page or contact us! We are always looking for ways to improve your experience.

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