Why Is My Check Engine Light Flashing? [Fixed!]

In this guide, we’ll be discussing why your check engine light might be flashing. If you’re like me, seeing that little warning light flashing can be panic-inducing, but don’t worry, we’ll go through the common reasons for this pesky problem and how you can fix it.

So, why is check engine light flashing? A flashing check engine light usually indicates a misfire in the engine, which can harm your engine over time. Common causes include a faulty oxygen sensor, ignition coil, spark plugs, fuel injector, vacuum leaks, loss of compression, malfunctioning mass airflow sensor, or clogged catalytic converter. To avoid costly issues, a diagnostic scan should be performed to identify the specific code(s) triggering the flashing light. Note that after disconnecting and reconnecting battery terminals, the check engine light may not be a serious concern as the ECU needs time to relearn engine conditions (around 20+ miles).

Also Read: Check engine light flashing but no codes

Difference Between A Steady Check Engine Light and A Flashing Check Engine Light

A steady check engine light indicates that there is a minor problem with the vehicle’s engine or emissions system, such as a loose gas cap. It may not require immediate attention, but the issue should be addressed soon to prevent further damage.

On the other hand, a flashing check engine light is a cause for concern as it indicates a severe problem with the engine or emissions system.

A steady check engine light is usually in the yellow color and a flashing check engine light blinks either in red or orange.

It’s important to note that a flashing check engine light is not something to ignore or dismiss.

In fact, driving with a flashing check engine light can cause serious damage to the engine and result in costly repairs.

Additionally, it can also be a safety hazard as it could cause the vehicle to suddenly stall or lose power while driving, potentially leading to an accident. 

Note: Both steady and flashing check engine light deals with the emission systems. Any problem in engine combustion can result in bad emissions. However, if you have just disconnected the battery to clean battery terminals or to change the battery, your vehicle would not pass the emission test until the ECU (engine control unit) resets. Due to this, the check engine light also comes on.

Flashing Check Engine Light and Engine Misfire

The check engine light is a warning light that appears on the dashboard of your vehicle to indicate that there is a problem with the engine or emissions system. It’s a part of the onboard diagnostics (OBD) system, which monitors a variety of engine functions and emissions levels to ensure that your vehicle is running properly. 

A flashing check engine light is a warning that indicates a serious problem with the engine that requires immediate attention.

Unlike a steady check engine light, which indicates a less urgent problem, a flashing check engine light signals that your engine is misfiring, which can cause damage to the catalytic converter and other engine components if left untreated. 

In addition, there are a few common symptoms that can indicate an engine misfire, including: 

• Rough idling or stalling
• A decrease in power or acceleration
• Increased fuel consumption
• A noticeable increase in exhaust emissions

To detect the exhaust trouble code that is causing the flashing check engine light, you need to plug the scan tool into the OBD2 port of the vehicle.

I would highly recommend the BlueDriver scan tool. It is quite affordable and has a range of features. It can easily connect with both Android and iPhone.

Usually, engine cylinder misfire codes are of the pattern ‘P030X‘, where X shows the cylinder number in which the misfire is occurring. If the code is P0301, it means that the misfire is occurring in the first cylinder. 

If you have no idea where to start counting engine cylinders, I have written this guide for you.

If you see the P0300 code, it means that the engine misfire is completely random and it is not specific to any cylinder. A loss of compression or poor combustion due to a bad oxygen sensor (lean or rich mixture) and vacuum leaks can cause this trouble code.

Causes Of Check Engine Light Flashing

Here are the causes of a flashing check engine light:

1. Fouled Spark Plugs

A spark plug is a small part that sits in the engine’s cylinder head. Spark plugs play a crucial role in the performance of your vehicle’s engine.

They are responsible for igniting the air-fuel mixture in the engine’s combustion chamber, which powers your car. When one or more of your spark plugs become fouled, it can cause your check engine light to flash.

Fouled spark plugs are spark plugs that have become coated or dirty, which prevents them from functioning correctly. 

Oil-fouled spark plugs occur when oil leaks into the combustion chamber, coating the spark plug and preventing it from firing.

When oil gets onto the spark plug, it can create a layer of insulation that makes it difficult for the spark plug to operate correctly.  This can be caused by a variety of issues, such as worn piston rings, valve stem seals or a faulty gasket.

Carbon-fouled spark plugs occur when carbon deposits build up on the spark plug’s electrodes, preventing it from producing a spark. This can be caused by a variety of factors, including low-quality fuel, short trips, and stop-and-go traffic. 

Carbon fouling can also occur if the engine is not running at the proper temperature, which can cause incomplete combustion.

When the spark plug becomes coated with carbon, it can no longer create a spark, which can lead to misfires and engine problems.

Apart from damaging the electrode tip of the spark plug, the ceramic insulator (white-colored part) of the spark plug can also become fouled.

The ceramic insulator of a spark plug serves as an insulator between the center electrode and the metal body of the spark plug. Its primary purpose is to provide electrical isolation and prevent the spark from grounding out before it can ignite the fuel mixture in the engine’s combustion chamber.

In simple words, the ceramic insulator of a spark plug serves as a dielectric barrier that prevents the electric current generated by the ignition coil from shorting out and reaching the engine block.

If the current were to reach the engine block, it would cause a short circuit and damage the engine’s electrical system. 

When the ceramic insulator of a spark plug becomes fouled with oil, or other contaminants can build upon its surface, which can cause an electrical leak across the insulator, leading to incomplete combustion of the fuel mixture in the engine and creating a misfire.

As a result of fouling, the spark plug cannot produce the required spark to ignite the fuel mixture in the engine, leading to hesitation, reduced power, and acceleration

Moreover, if your engine is leaking coolant into the combustion chamber, it can coat the spark plugs and prevent them from firing. This can be caused by a faulty head gasket, cracked cylinder head, or other issues.

How to spot?

To find out how the spark plug is fouled, you need to remove it using the spark plug socket. After that, visually inspect the spark plug to see how it was fouled.

You can take help from the spark plug trouble tracer chart (PDF download) to learn more.

How to fix?

The decision to clean or replace spark plugs depends on their condition. If the spark plugs are worn out or damaged, they should be replaced.

If they are still in good condition, they can be cleaned and re-gapped. Re-gapping of the spark plug means that the gap between the tips of the spark plug electrodes is re-adjusted based on the manufacturer’s recommendation.

The process of cleaning spark plugs depends on how they are fouled. First, use compressed air to clean off any dirt and debris.

Then, keep the spark plug soaked in the brake cleaner till all the gunk on the spark plug is washed away. You can also use a nylon brush to loosen the debris.

2. Bad Ignition Coil

An ignition coil is an electromagnetic device that converts the battery’s low voltage into the high voltage needed to create a spark in the spark plugs. 

There are two main types of ignition coils: coil over plug, coil pack and distributor.

Coil over plug ignition systems (COP), also known as direct ignition systems, use an individual ignition coil for each spark plug. This design eliminates the need for a distributor, which distributes the high voltage from a single ignition coil to each spark plug in a traditional ignition system. 

A distributor-based ignition system uses a single ignition coil to generate high voltage, which is then distributed to each spark plug via a distributor cap and rotor.

The distributor cap and rotor work together to ensure that the high voltage is delivered to the correct spark plug at the right time. 

The ignition oil is connected directly to the rotor, and the rotor spins inside the distributor cap. The rotor is attached to the shaft of the gear drive.

The gear drive is driven by the camshaft. On the underside of the distributor cap, a spring-loaded carbon brush contacts the metal part of the rotor.

The distributor shaft rotates when the camshaft rotates. As a result, the rotor attached to the distributor shaft also starts rotating.

As the outer edge of the rotor passes each internal plug terminal in the distributor cap, the spark plugs will fire in the correct order.

A bad ignition coil can cause a flashing check engine light because it can disrupt the engine’s firing order. When an ignition coil fails, it can cause a misfire in the engine, which means that the fuel and air mixture in one or more cylinders does not ignite properly. This can cause the engine to run rough, idle poorly, and even stall.

When the engine’s computer detects a misfire, it will trigger the check engine light to flash. This indicates that there is a problem with the engine that needs to be addressed immediately.

If the problem is not fixed, it can cause damage to the engine and other components in the vehicle due to excessive vibrations.

The coil pack ignition system is another type of ignition system that uses a single coil to fire multiple cylinders. In this system, the coil pack is mounted on the engine and is connected to each spark plug via a spark plug wire. 

How to spot?

If your vehicle has COP, the best way to check bad ignition coils is to swap the ignition coil with another cylinder’s coil and see if the trouble code changes.

Another way to check a bad ignition coil is that if you remove the ignition coil of a certain cylinder and the engine starts to stumble, it means the ignition coil is good. If engine stays same, it means the ignition coil is bad.

If your vehicle has a distributor, the contacts inside the distributor cap can become corroded or worn down, which can lead to a weak or intermittent spark. The rotor can also wear down over time, which can cause it to make poor contact with the contacts inside the distributor cap.

For ignition coil pack, follow the below video:

3. Bad Oxygen Sensor

There are typically two oxygen sensors in a car’s engine management system. The first sensor is located upstream of the catalytic converter and the second sensor is located downstream of the catalytic converter.

The first oxygen sensor is responsible for measuring the amount of oxygen in the exhaust gases before they enter the catalytic converter.

The second oxygen sensor measures the amount of oxygen in the exhaust gases after they have passed through the catalytic converter. 

When the first oxygen sensor detects a high amount of oxygen in the exhaust gases, it sends a signal to the car’s engine control module (ECM).

The ECM then adjusts the fuel injection system to add more fuel to the engine. This is because a high amount of oxygen in the exhaust gases indicates that there is not enough fuel being burned in the engine. 

If the first oxygen sensor is not functioning properly, it may send a false signal to the ECM indicating that there is too much or too little oxygen in the exhaust gases.

This false signal can cause the ECM to adjust the fuel injection system incorrectly, which can lead to a number of problems, including lean or rich air-fuel mixture, reduced fuel economy, reduced engine performance, and increased emissions. 

If the ECM detects that the first oxygen sensor is sending inconsistent or incorrect signals, it may cause the check engine light to flash.

This flashing check engine light is a warning to the driver that there is a serious problem with the engine management system and that the car should be taken to a qualified technician for repairs. 

Why does it happen?

There are several things that can cause an oxygen sensor to fail. One of the most common causes is contamination. The oxygen sensor can become contaminated with oil, coolant, or other substances, which can cause it to malfunction.

Another common cause of oxygen sensor failure is age. Over time, the sensor can become worn out and less effective at measuring the amount of oxygen in the exhaust gases. 

How to fix?

The voltage generated by the oxygen sensor is expected to vary as the air-fuel ratio fluctuates. However, if the oxygen sensor malfunctions and the voltage does not vary as expected, the engine may not perform at its optimal level.

It is expected that in a proper functioning O2 sensor, the voltage fluctuation should occur at least once every two seconds between 150mV and 850mV.

In case the O2 sensor’s reading gets stuck at a particular position or the voltage readings are unusually high or low, it is an indication that the O2 sensor has failed.

To test the O2 sensor, you should connect the positive lead of the voltmeter to the signal wire of the O2 sensor. If the O2 sensor has two wires, connect the negative lead to the negative wire of the sensor.

Replacing an oxygen sensor is a relatively simple and inexpensive repair. However, if you ignore the problem, it can lead to more serious issues down the road. 

4. Damaged Catalytic Converter

One possible reason for a flashing check engine light is that the catalytic converter is not working efficiently. This means that it is not converting the pollutants in the exhaust as effectively as it should be. When this happens, the engine control module (ECM) in your car will detect that there is a problem and will turn on the check engine light. 

A catalytic converter is a device that is installed in a vehicle’s exhaust system. It is responsible for converting harmful gases into less harmful ones before they are released into the environment.

The catalytic converter works by using a chemical reaction to break down pollutants such as carbon monoxide and nitrogen oxides into less harmful substances like water vapor and carbon dioxide.

Catalytic converters have precious metals employed as catalysts, namely platinum, palladium, and rhodium coated on a ceramic honeycomb substrate. The purpose is to convert harmful pollutants into less toxic substances that are more eco-friendly.

In addition to the catalyst, the metal casing surrounding the core of the catalytic converter is a crucial component. It’s responsible for leading the exhaust gas stream over the catalyst bed. Generally, stainless steel is the preferred material for this metal casing.

Why does it happen?

If the converter becomes clogged with unburnt fuel or its ceramic honeycomb structure is damaged, it can’t perform its function properly and the exhaust gases won’t be able to escape the cylinder before the new air/fuel charge comes in the next cycle.

This is because the carbon and crud buildup of unburnt fuel gets stuck in the honeycomb pattern of a catalytic converter and damages it.

This can cause a backup of exhaust gases within the cylinder, meaning the engine can’t properly expel the burnt gases from the combustion process. This results in a reduction of power as well as an increase in fuel consumption, which ultimately leads to engine misfires, reduced acceleration, and increased emissions.

How to spot?

To test a bad catalytic converter, you can use a laser thermometer.

• Start the engine for a few minutes.
• Keeping the engine running, pop up the hood and point out the thermometer at a point where exhaust gases enter the catalytic converter.
• Now, under the vehicle, point out the thermometer at a point where gases exit the catalytic converter.
• If there is a significant difference between both temperatures, it means that exhaust gases are being trapped in the catalytic converter, and are not exiting properly.

5. Vacuum Leaks

In simple terms, vacuum leaks are unintended openings in the engine’s intake manifold or vacuum hoses. These openings allow unmetered air to enter the engine, causing the air-fuel mixture to become lean.

A lean mixture can cause several issues with the engine, including misfires, rough idle, and poor performance.

Now, you might be confusing how a vacuum is produced in the engine…

When the engine is running, it creates a vacuum that draws in air, fuel, and exhaust gases. This vacuum is created by the downward movement of the pistons during the intake stroke.

The vacuum system is responsible for regulating the amount of air that enters the engine.

This is done by controlling the opening and closing of the intake valves, which allow air and fuel to enter the cylinders.

When a vacuum leak occurs, it allows unmetered air to enter the engine, which disrupts the air/fuel ratio and can cause a misfire.

Now, you might also confuse about the term “unmetered air.” Before the throttle body, there is a sensor that calculates the amount of air entering the engine cylinder.

If the air leaks into the engine at anywhere after the sensor, it will not be able to measure that air and send signals to the ECM.

As a result, the ECM will not be able to adjust the fuel accordingly. This will disturb the air-fuel mixture and cause flashing check engine light.

How to spot?

If you suspect that your car has a vacuum leak, look out for the following signs:

1. Rough idle: A vacuum leak can cause the engine to idle roughly or even stall.
2. Reduced power: If your car doesn’t have the same power as it used to, it could be due to a vacuum leak.
3. Check engine light: A vacuum leak can trigger the flashing check engine light to come on.
4. Hissing noise: A vacuum leak can create a hissing noise under the hood.

There are several ways in which vacuum leaks can occur in an engine: 

• Damaged Hoses: The vacuum system in an engine is made up of a series of hoses that connect the various components of the system. Over time, these hoses can become damaged or cracked, which can cause leaks.
• Faulty Gaskets: Gaskets are used to seal various parts of the engine, like the intake manifold and throttle body. If these gaskets become worn or damaged, they can allow air to enter the engine, causing a vacuum leak.
• Loose Connections: The vacuum system in an engine relies on tight connections to function properly. If any of these connections become loose, air can enter the system, causing a leak.
• Broken or Cracked Components: The various components of the vacuum system, like the PCV valve or brake booster, can become damaged over time, causing leaks.

You can use carb cleaner to detect vacuum leaks in the engine.

6. Loss Of Compression

In a gasoline engine, compression is the process of compressing the air/fuel mixture in the cylinder before it’s ignited by the spark plug. This compression creates the pressure needed for the fuel to ignite and power the engine.

The compression stroke compresses the air and fuel mixture in the cylinder, which is then ignited by the spark plug to produce power.

If there’s a loss of compression, the engine’s power output will decrease, and it will run poorly. As a result, the fuel will not burn efficiently, a misfire will occur, and the check engine light will start flashing.

There are a few different things that can contribute to a loss of compression, including:

1. Worn piston rings: Piston rings are responsible for sealing the combustion chamber and preventing the air/fuel mixture from leaking out. Over time, the piston rings can wear out and lose their ability to seal the chamber, leading to a loss of compression. This can be caused by a variety of factors, including poor maintenance, high mileage, or overheating.
2. Damaged cylinder walls: The cylinder walls provide a smooth surface for the piston rings to slide against. If the walls become damaged, the rings can no longer seal the combustion chamber, leading to a loss of compression. This can be caused by a variety of factors, including overheating, poor lubrication, or dirty oil.
3. Leaky valves: The valves control the flow of air and fuel into and out of the combustion chamber. If the valves are damaged or worn, they can leak air/fuel mixture out of the chamber, leading to a loss of compression. This can be caused by a variety of factors, including poor maintenance, overheating, or dirty oil.
4. Blown head gasket: The head gasket seals the cylinder head to the engine block and prevents the air/fuel mixture from leaking out. If the head gasket blows, it can allow the mixture to leak out, leading to a loss of compression. This can be caused by a variety of factors, including overheating, high mileage, or poor maintenance.
5. Cracked cylinder head or block: If the cylinder head or block becomes cracked, it can allow the air/fuel mixture to leak out, leading to a loss of compression. This can be caused by a variety of factors, including overheating, poor maintenance, or manufacturing defects.
6. Bad timing chain/belt: The timing chain synchronizes the opening and closing of engine valves. If the chain is worn or stretched, it can cause the valve timing to become off. This means that the engine’s valves open and closing at the wrong time, which can cause the engine to lose compression, which can lead to a loss of compression.

To learn more about head gasket failure and leaking of valves, you can read my guide on car won’t start after overheating.

How to spot?

Follow these steps to perform a compression test to detect loss of compression in engine:

1. First, disconnect the electric connectors of the fuel pump, fuel injectors and ignition coils/distributor.
2. Remove all spark plugs. Use a suitable spark plug socket to remove spark plugs.
3. For the first cylinder, screw the compression gauge in the hole where the spark plug goes.
4. Turn on the ignition and depress the accelerator pedal fully to keep the throttle plate open.
5. Crank the engine up to 5 times or do it until the needle on the compression gauge is peaked and doesn’t climb anymore. If you have a push start/stop button, press brake pedal and push the button multiple times.
6. Note down the pressure reading for each cylinder on a piece of paper. If you don’t know how cylinders are numbered in the engine, you can read this guide.
7. If one of the pressure readings is significantly lower, it can indicate a problem with that single cylinder. 
8. If you only have a low reading on one engine cylinder or the cylinders aren’t adjacent to each other, the issue is with the valve seal or piston ring. 
9. If adjacent cylinders have low-pressure readings, the issue might be with the head gasket.
10. When all your pressure ratings inside the engine cylinders are below 100 PSI for gasoline engines or below 275 for diesel, you may have a valve timing issue, or piston rings are worn-out.
11. To determine worn-out piston rings, a wet compression test is performed. The wet compression test involves adding a small amount of oil through a spark plug hole to see if the compression reading improves, which can help determine whether the low compression is due to piston rings or valves.
12. If the compression reading improves, it suggests that the low compression reading was due to worn-out piston rings. When oil is added to the cylinder, it acts as a lubricant and seals off the gaps between the piston rings and cylinder walls, leading to improved compression readings.

Final Thoughts On Check Engine Light Flashing

In summary, a flashing check engine light is a sign of a serious issue that requires prompt attention. Common causes include spark plug, ignition coil, oxygen sensor, or catalytic converter faults leading to misfires.

Vacuum leaks and loss of compression can also trigger the flashing light. Diagnostic scanning is key to pinpoint the trouble code(s). Addressing the underlying issue quickly can prevent further engine damage and costly repairs down the road.