If your check engine light is flashing and your car is shaking, you’ll want to diagnose your car for a misfire. This is the most common reason a check engine light will flash and cause a rough running engine that can lose power. In this article and video, we review what can cause a flashing check engine light and a shaking sensation in your car, and how to tell if a misfire is causing this yourself by diagnosing the ignition system.
Why Is My Check Engine Light Flashing and My Car Shaking?
Bad Spark Plugs or Ignition Coils Causing a Misfire
Faulty spark plugs and ignition coils are the most common reason for a running rough engine that causes shaking and a flashing check engine light. The shaking is usually from a misfire, and it’s common for the check engine light to flash for ignition-related codes. You might also notice a reduction in power.
What Should I Do If My Check Engine Light Is Flashing and My Car Is Shaking?
If you have a flashing check engine light and notice shaking, it’s best to stop driving and pull over to a safe area to prevent damage to other parts.
How to Diagnose the Ignition Coils and Spark Plugs
The engine used in this example is a 4 cylinder, which is easier to diagnose. We also have tips on how to diagnose a 6 cylinder engine.
Steps to diagnose the cause of a shaking car and blinking check engine light
- Scan for Codes with a Scan Tool
Take a scan tool and scan for any troubleshooting codes. You may find you have a code like P0300, which is a general misfire code. You may also find codes related to a specific cylinder. The number at the end of the P0300 code indicates a specific cylinder, like cylinder 3 for P0303. If there was a check engine light on for code P0303, this would mean one cylinder, specifically cylinder 3, is misfiring, and you will only have 7 working cylinders if your engine is a V8, 5 working cylinders if your engine is a V6, etc.
You may also find you have two cylinders not working, or you may pull a code for a random misfire.
Some scan tools, like the one featured in this video, will allow you to graph misfires and can help you save time and narrow down which cylinder is misfiring. We’ll review some techniques you can try if you do not have a scan tool that can graph.
- Check the Ignition Coils
The engine will need air, fuel, spark, and compression to run properly. It’s easiest is to check for spark. We’ll look into whether the issue is with the ignition system or coil and spark plug.
To do this, you’ll need access to the coil and spark plugs, and you may have to remove other parts to do this, like an engine cover or an intake manifold. You’ll also want to find the engine cylinder order so you can identify the different cylinders.
This is a coil-over-plug set up which is a common system in today’s cars. The computer sends a signal to the ignition coil, which sends a high electrical voltage to the spark plug. The spark in the spark plug then ignites the fuel and creates the combustion needed to get the engine working correctly.
In this example, all of the ignition coils and spark plugs are the same design, so one test you can try by disconnecting the ignition coil and swapping it with another to see if the misfire follows.
Sometimes the ignition coil boot can rip. Our mechanic removed the ignition coil and the rubber boot remained stuck inside the cylinder, which may be the problem. The boot can stick if dielectric grease is or was not added.
In our case the misfire followed the ignition coil, so we know the problem is the ignition coil and we’ll be replacing it.
More on testing a bad ignition coil
If it’s been some time since the coils have failed, we recommend replacing all of the coils and spark plugs. Ignition coils are a wear part, and if it’s been in use and is faulty, the others probably aren’t far behind.
The spark plug could have caused coil to fail. If there’s too large of a gap on the spark plug or the spark plug is faulty, the coil will work harder and overheat, causing failure.
- Check the Spark Plugs
If the misfire did not follow the ignition coil, check the spark plugs next. Remove the ignition coil. Check the spark plug well for debris or fluids like coolant or oil, which could also be the cause of a misfire.
More on finding oil in the spark plug well
Check the condition of the spark plug. Inspect it for excessive carbon build-up. If the ground tab is missing, that’s also not a good sign and means there’s probably some other kind of mechanical damage.
If the spark plug gap is closed, maybe it was pushed down or dropped, or maybe the gap was affected and not resized. Regardless, this will also be noticeable and won’t happen after long periods of driving.
Check the porcelain and look for signs of arching out. This means it’s not sealing properly. Since there needs to be a seal, there could be an issue with the boot. If there’s no seal, the spark isn’t going to the spark plug, but coming out of the coil and arcing and grounding out. This can create a black line that looks similar to electricity.
Then swap the spark plug with one in another cylinder and see if the misfire follows. Put the spark plug in by hand and then tighten it. If the misfire does follow the spark plug, it needs to be replaced. We recommend replacing all of the spark plugs, since most are going to fail.
In this case, the electrode was touching the ground. If the misfire still doesn’t follow, then the ignition coil and spark plug are probably not the cause of the misfire. You may have problems with the fuel injector or something mechanical that’s not giving you compression, like the piston or valves not sealing properly.
If you do not have a code for a specific cylinder but a general P0300 misfire code and you don’t have a scan tool that can graph the misfire, you can connect an inline spark tester. Connect one end to ignition coil and one end to the spark plug. Then start vehicle and see if you have spark. Don’t touch anything near it or you could ground it out or hurt yourself. You can do this for each coil and see if lights up. If it doesn’t light up while running and you don’t see a spark from the coil, then you know its faulty.
How to Check a Misfire with a Scan tool that can Graph
It’s easier to see a misfire with a scan tool that can graph. You can select the individual cylinders. Then start the vehicle and you can see how many misfires occur.
When installing a new spark plug and ignition coil, it’s good practice to check the gap before installing the spark plug.
If your engine has more cylinders, it will be harder to diagnose the ignition coil. There’s usually an intake plenum that needs to be removed or loosened and moved to the side. You may have to remove other parts like the air intake hose to access the ignition coils on the backside
You’ll also want to check the firing order of the cylinders. Some alternate the firing order, and it depends on the vehicle. For a V6, move both the coil and plug and see if the misfire follows so you know it’s one of the two. Then you can work from the front and diagnose the coil or plug further, or find out what else is causing the misfire.