Why Is My Check Engine Light Flashing?

A blinking check engine light often means a misfire in the engine. This can harm your motor over time. Usual reasons are an oxygen sensor not working right, ignition coil going bad, spark plugs failing, fuel injector problems, vacuum leaks, loss of compression, mass airflow sensor not working right, or catalytic converter getting clogged up. To avoid expensive repairs, you should get a diagnostic scan. This will identify the exact code making the blinking light happen. Note that after disconnecting and reconnecting the battery terminals, the check engine light might not be a big deal. The ECU needs time to relearn engine conditions (around 20+ miles).

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.

Sometimes check engine light also flashes without showing any trouble codes. You can read my guide on that topic.

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

difference between steady and blinking check engine light

The steady check engine light shows a minor issue with the vehicle’s engine or emission system, like a loose gas cap. It might not need quick attention, but you should get it checked soon so more damage doesn’t happen.

On the other hand, the flashing check engine light is worrying since it means a big problem with the engine or emission system.

The steady check engine light is usually yellow, and the flashing check engine light blinks red or orange.

It’s essential to note that the flashing check engine light isn’t something to dismiss.

In fact, driving with the flashing check engine light could seriously hurt the engine and need expensive fixes.

Also, it might be dangerous since it could make the vehicle suddenly stall or lose power while driving, possibly causing a crash.

Note: Both steady and flashing check engine lights relate to the emission systems. Any issue with engine combustion can cause bad emissions. But, if you just disconnected the battery to clean the terminals or change the battery, your vehicle wouldn’t pass the emission test until the ECU (engine control unit) resets. That’s why the check engine light comes on too.

Flashing Check Engine Light and Engine Misfire

The check engine light is a warning on the dashboard. It shows if the engine or emissions system has a problem. The light is part of the onboard diagnostics (OBD) system. This system checks the engine and emissions. It makes sure your vehicle runs right.

A flashing check engine light means a big problem. It needs quick fixes.

A steady light means a smaller issue. But flashing means the engine misfires. This can hurt the catalytic converter. It can damage other engine parts too if not fixed.

Some signs of an engine misfire are:

  • Rough idling or stalling
  • Less power or slower acceleration
  • Using more fuel
  • More exhaust

To see the code for the flashing light, use a scan tool. Plug it 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.

scanning misfire trouble codes using OBD2 tool

Engine trouble codes starting with “P030X” point to cylinder misfires, with the X showing the misbehaving cylinder. So P0301 means cylinder #1 is acting up.

But where do you start counting to find #1? No worries. I’ve written this guide on identifying engine cylinder numbers to help you locate and make sense of your engine’s cylinders.

Now a P0300 code is sneaky – it means a random misfire with no particular cylinder at fault. The culprits? Often loss of compression or a lean or rich fuel mix from a troubled oxygen sensor. Vacuum leaks can also cause the random misfire code.

Causes Of Check Engine Light Flashing

Here are the causes of a flashing check engine light:

1. Fouled Spark Plugs

Two fouled oxygen sensors with visible deposits on their surfaces, a common cause for a flashing check engine light due to sensor malfunction.
fouled spark plugs

The spark plug sits small inside the engine’s cylinder head. It has a big job of igniting the air-fuel mix in the combustion chamber. This powers the car. When one or more spark plugs get dirty, it can make the check engine light flash.

Dirty spark plugs are plugs coated in gunk. This stops them working properly.

Oil-dirtied spark plugs happen when oil leaks into the combustion chamber. It coats the plug so no spark fires. The oil makes insulation on the plug. This makes it hard to work. Worn piston rings, valve seals or a bad gasket can cause this oil leak.

Carbon-dirtied spark plugs happen when carbon builds up on the electrodes. This stops the spark. Low-grade fuel, short trips, and stop-go traffic can cause this.

The carbon stops the spark for lighting the air-fuel mix. This brings misfires and problems.

The white ceramic part of the spark plug can also get dirty. It insulates between the electrode and the metal body of the spark plug. It stops the spark from grounding out before it can ignite the fuel mixture in the engine’s combustion chamber.

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

If oil or gunk coats the ceramic, it can make a leak across it. This means incomplete burning in the engine. The spark plug can’t spark to ignite the air-fuel mixture. This brings hesitation, less power and slow acceleration.

Also, coolant leaks can coat the spark plugs. This stops them from sparking. A bad head gasket, cracked head or other issues can cause the coolant leak.

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?

Cleaning or replacing spark plugs depends on their condition. Spark plugs that are worn out or damaged need replacing.

Spark plugs in good condition can get cleaned and re-gapped. Re-gapping the spark plug means readjusting the gap between the electrode tips as per the manufacturer’s recommendation.

The cleaning process depends on how the plugs got fouled. First, blast compressed air to remove any dirt and debris.

Then, soak the spark plug in brake cleaner until all gunk on it washes away. A nylon brush also helps loosen stuck debris.

2. Bad Ignition Coil

ignition coil types

An ignition coil is a device that changes the battery’s low power to the high power needed to make a spark in the spark plugs.

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

Coil over plug ignition systems (COP), also called direct ignition systems, use one ignition coil for each spark plug. This design removes the need for a distributor, which sends the high power from one ignition coil to each spark plug in an old ignition system.

A distributor ignition system uses one ignition coil to make high voltage, which a distributor cap and rotor then send to each spark plug.

The distributor cap and rotor work together to ensure the high voltage reaches the right spark plug at the right time.

The ignition coil connects directly to the rotor, and the rotor spins inside the distributor cap. The rotor attaches to the shaft of the gear drive.

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

The distributor shaft turns when the camshaft turns. So the rotor on the distributor shaft also starts spinning.

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

A bad ignition coil can cause the check engine light to flash because it can mess up the engine’s firing order. When an ignition coil fails, it can cause a misfire in the engine. This means the fuel and air mix in one or more cylinders does not ignite right. This can make the engine run rough, idle poorly, and even stall.

When the engine’s computer finds a misfire, it will make the check engine light flash. This shows there is an engine problem that needs fixing right away.

If not fixed, it can damage the engine and other parts in the vehicle from too much shaking.

The coil pack ignition system is another kind of ignition system that uses one coil to fire multiple cylinders. In this system, the coil pack is mounted on the engine and connects to each spark plug through 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.

Inside view of a worn distributor cap with corroded contact points, which could be responsible for a flashing check engine light due to ignition issues.

For ignition coil pack, follow the below video:

3. Bad Oxygen Sensor

Hand holding a faulty oxygen sensor with apparent damage, likely to cause a flashing check engine light due to sensor failure.

The car engine has two oxygen sensors. One sensor is before the catalytic converter. The other sensor is after.

The first oxygen sensor checks the oxygen amount in the exhaust gases before they reach the catalytic converter.

The second oxygen sensor checks the oxygen amount after the gases pass through the converter.

When the first oxygen sensor sees high oxygen in the exhaust, it tells the engine control module. The module is called ECM.

The ECM then adjusts the fuel injection. It adds more fuel to the engine. High oxygen means not enough fuel is burning.

If the first oxygen sensor malfunctions, it may tell the ECM the wrong oxygen amounts.

This makes the ECM adjust fuel injection wrong. It can cause problems. Lean or rich air-fuel. Lower fuel economy. Lower engine power. More emissions.

If the ECM sees the first oxygen sensor is malfunctioning, it can flash the check engine light.

Why does it happen?

The oxygen sensor tells the engine if the air and fuel mix is rich or lean. Some things can make it not work right.

One big thing is gunk. Oil, coolant, or other stuff can get on the sensor and cause it to malfunction.

Another thing is the oxygen sensor just gets old. After some years, it wears out. Then it can’t measure the oxygen in the exhaust like it should.

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

catalytic converter schematic

The check engine light flashing can mean the catalytic converter is not working properly. This part cleans up exhaust before it goes out the tailpipe. When the catalytic converter doesn’t work like it should, the car’s computer sees a problem. It turns on the check engine light.

The catalytic converter is in the exhaust system. It changes toxic gases into less harmful gases. That way they don’t hurt the air as much when they come out of the car.

The catalytic converter uses a chemical reaction. This breaks down pollutants like carbon monoxide and nitrogen oxides. It turns them into less harmful things like water vapor and carbon dioxide.

Precious metals help the reaction happen. Platinum, palladium, and rhodium coat a ceramic honeycomb inside the converter. They help turn the pollutants into less toxic stuff better for the environment.

The metal case around the ceramic core matters too. It makes the exhaust flow over the catalyst. Stainless steel works best for the 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

The air-fuel mixture becomes lean when unintended openings allow uncalculated air into the intake. These openings are vacuum leaks.

When extra air enters, the ratio of air to fuel is disturbed. This can cause misfires, rough idling, and poor performance.

The engine makes a vacuum as the pistons move down. This vacuum pulls in the air, fuel, and exhaust gases needed. The amount of incoming air is controlled by valve opening and closing.

Extra air from a leak disrupts the ratio. The engine computer can’t adjust the fuel correctly. This causes a check engine light.

The MAF sensor before the throttle body measures the air. If air enters after that point, the computer can’t know. So it can’t fix the fuel amount to match.

The engine needs the right air-fuel mix. Extra air causes problems. So leaks must be fixed.

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

engine compression loss demonstration

The squeezing of the air/fuel mix in the cylinder before the spark lights it is called compression. This squeezing makes the pressure needed for the fuel to burn and power the engine.

The compression stroke squeezes the air and fuel in the cylinder. Then the spark plug ignites it to make power.

If there is a loss of compression the engine’s power will go down. It will run poorly. So the fuel won’t burn well. A misfire will happen. The check engine light will start flashing.

Some things can cause a loss of compression. They are:

  • Worn piston rings: The rings seal the burning chamber. They keep the air/fuel from leaking out. Over time the rings can wear out. They lose their sealing ability. This causes a loss of compression. It can happen from poor vehicle maintenance, high mileage or overheating.
  • Damaged cylinder walls: The walls give a smooth surface for the rings to slide on. If the walls get damaged, the rings can’t seal the chamber anymore. This leads to lost squeezing. It can happen from overheating, poor lubrication or dirty oil.
  • Leaky valves: The valves control the air and fuel flow in and out. If the valves are damaged or worn, they can leak the air/fuel out. This causes a loss of compression. It can happen from overheating or dirty oil.
  • Blown head gasket: The gasket seals the cylinder head to the engine block. It keeps the air/fuel from leaking out. If the gasket blows, it can let the mix leak out. This leads to lost compression.
  • Cracked cylinder head or block: If the head or block cracks, it can let the air/fuel leak out. This also causes a loss of compression.
  • Bad timing chain/belt: The timing chain syncs the valve opening and closing. If the chain wears or stretches, the valve timing gets off. The valves open and close at the wrong time. This can make the engine lose compression ratio.

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.

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