Symptoms of Bad Spark Plugs

Spark plugs are one of the most important parts of any gas-powered engine. Basically, they will send the electrical signal from the coil to the inside the cylinder at a specific time and ignite the air and fuel mixture. How long do spark plugs last? They can last between 30.000 and 100.000 miles, depending on your vehicle and the spark plugs themselves. Eventually, they will fail. When this happens you need to diagnose the problem. Here we will list the main symptoms on how to tell if a spark plug is bad. The fix is easy. Simply replace the spark plugs.

Symptoms of Bad Spark Plugs

There is no single symptom that will help you learn how to tell if a spark plug is bad. In other words, you might experience 2 or more symptoms and each one is different. Below are all the symptoms your car may have.

what happens when spark plugs go bad

Decreased acceleration

One of the most f is the decreased acceleration. This happens when the efficiency of the spark plug is decreased. It consists of multiple materials and all of them must work properly to send the electrical signal to the correct moment to the engine. Eventually, this will decrease, meaning that the spark plug won’t send the signal at all or it will send it slower than needed.

When there is no signal sent, the cylinder won’t work. Your car will literally work on 3 or even fewer cylinders. When the efficiency is decreased, a spark plug may send an electrical signal at the wrong moment or in lower voltage which makes the cylinder unable to produce power, hence the decreased acceleration.

We must add that this issue can happen due to a damaged fuel injector, a sensor on your camshaft or crankshaft, and even by the clogged fuel filter. It is a smart move to start by replacing the spark plugs, especially if they are old. In some cases, you may have to replace the spark plug wires as well.

Higher fuel consumption

This symptom is common and usually linked to failing spark plugs. So why does this happen? When the spark plug is affected and its efficiency decreased, it won’t be able to ignite the fuel and air mixture properly. Your car will need more fuel and it will increase fuel consumption accordingly.

The symptom can occur due to too big or too small a gap between the electrode and the central piece on a spark plug. The gap is pre-determined by the manufacturer and it is measured in millimeters. Each car has a different value so you will need a tool to measure that gap and adjust it. Replacing the spark plugs will do the trick as well.

Misfiring of the engine

This is the most common issue linked to the wiring of a car. Misfire means that the mixture of air and fuel in a cylinder won’t be ignited at the correct time. You will hear the engine literally working irregularly and you can hear explosion sound from the exhaust.

A misfire means that the electrical signal from the coil won’t be sent at the correct time to the sparkplug, or a spark plug won’t transfer it in the correct moment. It can be sent while the piston is traveling up in the cylinder or when it is returning down. Either way, you will notice much higher fuel consumption, issues starting the engine, and decreased performance.

Keep in mind that a misfire can occur due to the damaged or faulty sensor on the camshaft or crankshaft. But, if your car has an onboard computer it will show a corresponding error code.

Surging engine

Here we can see one symptom that is linked to a bad spark plugin 99% of cases. It is very easy to notice. While driving your car you will notice a loss of power and decreased performance. This happens when one or more spark plugs don’t work. When you press the throttle pedal and increase RPMs a spark plug will suddenly start to work. This will cause your car to get more power than it had 2 seconds ago.

This symptom is common in failing spark plugs. At some RPMs, they don’t work, but at higher, when the electrical signal rate increases a spark plug will start to work. Yes, it isn’t something you will want in your car and it isn’t safe. It can cause an accident or you can damage the engine.

Vibrating sound while idle

While idle your engine needs to be smooth and quiet. It means that all the components are working as they should. But, when a spark plug is failing, you will notice the vibrations and rough idle. The engine works as it should and you can see the just mentioned vibrations. The symptom is common when there are one or more issues with a spark plug.  It won’t ignite the fuel and air mixture when it should, so the engine will develop certain vibrations.

This isn’t the same symptom as misfire but it looks identical. A misfire is a more severe issue that has more negative effects and needs to be sorted out as soon as possible. This one, on the other hand, is a minor symptom and it means that some of your spark plugs are still fully operational.

Your car has issues starting the engine

If you have a hard time starting the engine, especially when it is cold outside spark plugs need to be replaced. We can add that this symptom is one of the first to appear but doesn’t mean that a spark plug has failed. It means that a spark plug or spark plugs are failing and they are unable to transmit the electrical signal as they should.

This is the first symptom of a more complicated issue. Your car will have more and more issues starting and eventually, the engine won’t start at all. It is always a smart idea to check the spark plugs and the spark plug wiring to make sure all of them are working properly. Once you have replaced the spark plugs you will see a major improvement and your car will start perfectly every time.

In addition, you may wonder what bad spark plugs look like. The most common sign is damaged curved part, damaged electrode, or carbon composite. You can see that the inner space around the electrode is clogged and the entire spark plug looks dirty. Don’t forget that the engine that ‘’consumes’’ oil due to damaged piston rings or rubber seals on the valves is more likely to damage the spark plugs over a shorter period of time. Oil doesn’t burn as clean as the fuel so the composites will affect the electrode of spark plugs.


Now you know how to tell if a spark plug is bad. Regardless of which symptom you have or how many, you need to check and replace the spark plugs. In many cases, a simple fix will do the trick and your car will operate again as it should. Try to remember that spark plugs are the smallest component of your car that has huge importance. As such, it isn’t difficult to understand that investing in high-end units is a wise investment. They will last longer and decrease the risk of any of these symptoms.


Hi there, I am R. Hasan Tito, a mechanic, and owner of this website. My friend and I created this website to share our knowledge, expertise, and experience with our fellow mechanics' community and car users. I am a specialist and certified automotive mechanic (Both Heavy Commercial and Private Cars). I worked as a Mechanic and Mechanic Supervisor for over fifteen years at Global Rebound Automotive companies - Toyota, TATA, BMW, Nissan, TVs, and Others. Now, I enjoy my new role of leading a team of automotive experts (in their respective fields) and publish new content on a regular basis on my website and social media.

4 thoughts on “Symptoms of Bad Spark Plugs

  1. Hi Tito , I really appriciate the things I learn through your Web sight. Very useful for all the car owners . Hoping to learn more and more Thanks

  2. I really enjoy and learn the things that I did not know before. Very useful for all the car owners . Hoping to learn more and more , Thanks imensely .

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