Are you wondered about what is it that causes the air/fuel mixture to ignite inside the combustion chamber? It is the spark plug that creates electricity to ignite the air/fuel mixture which starts the vehicle and also keep it running with continuous supply of power.

For the best performance of your engine, all the spark plugs must be in good condition. If any of the spark plugs gets damaged or fouled, the effect of this bad spark plug, you can experience in how the engine’s performance keeps degrading.

Sometimes, it can cause the motor not to start at all.

But thankfully, there are some symptoms that indicate you of bad spark plug and help you prevent the problems that can damage your engine.

Here, in this article, we will learn everything about the spark plugs. Let’s begin with the most simple but most important question

What is a Spark Plug?

A spark plug is a very small yet very important component of the ignition system. It is responsible for creating the spark that ignites the air/fuel mixture and makes the engine to produce power.

The spark plug mainly consists of four parts. First is its threaded metal shell which helps it to fit inside the engine cylinder head. There are two electrodes center electrode and ground electrode which are isolated with the help of insulator.

The terminal of the spark plug is connected to the ignition point from where the electricity is transmitted to the spark plug which passes through the center electrode.

As soon as the spark plug delivers this electric current from the Ignition system to the cylinder head, it uses the compressed fuel and the air mixture to create the spark, and continues to contain the pressure within the engine chamber.

Signs & Symptoms of a Bad Spark Plug

Generally, the spark plugs are made from extremely stiff and durable material so as they can withstand millions of explosions. But over time they wear out and reduce the efficiency of the engine along with a couple of other issues.

If you look out for symptoms of a bad spark plug, you can know when you need to replace them.

#1 – Slow Acceleration

You probably know how your vehicle drives, if you feel like it does not respond as it used to particularly when you step on the gas pedal, the failing spark plug could be the culprit.

The failing spark plug leads to a weaker spark that means improper burning of fuel/air mixture. This leads to a loss of engine power that causes poor acceleration.

While sluggish acceleration can be due to a number of reasons, it is best to visit a professional mechanic for a proper diagnosis of the issue.

#2 – Check Engine Light Turns On

The faulty spark plug may cause the check engine light to turn on. Generally, this is due to the catastrophic misfires takes place. The engine control unit recognizes this misfiring and turns the check engine light on with a specific trouble code.

There are several other reasons that cause the check engine light to come on, but if you read the trouble code, you can exactly know where the problem is.

Recommended Article: How to Reset Check Engine Light

#3 – Bad Fuel Economy

The fact is that a suitable spark plug helps to burn the fuel very efficiently. This would, in turn, promote the combustion cycle, and the vehicle would have the best fuel economy.

But due to wear and tear, the gap between the electrodes in the spark plug would either too close or too widen which affects the functioning of the spark plug. This result into loss of ability to burn fuel efficiently that leads to poor gas mileage.

Therefore, if you notice sudden increase in fuel consumption, you must get inspected the spark plug.

#4 – Engine Misfires

Whenever, there is incomplete burning of fuel in the combustion chamber, the engine misfiring is quite obvious. The main reason for improper burning of fuel is either failing spark plug or faulty sensor.

If you notice the sputtering sound from the engine, you should get inspected both the sensor and the spark plug.

Recommended Article: Symptoms of Car Engine Misfire

#5 – Rough Idle

When the vehicle is on idle and the RPM keeps dropping, this can be largely due to a faulty spark plug.

In this case, the sound from the engine seems rough and the vehicle vibrates too much. If you notice all of these signs together, then certainly the spark plug needs replacement.

#6 – Hard to Start

If your vehicle takes longer to start, there is possibility that there is some problem in sparking an ignition. This is largely due to a faulty spark plug. But before you go for the spark plug replacement, you must check the battery as it can also be the reason for hard starting of engine.

#7 – Strong Fuel Smell from Exhaust

Now, you know that the bad spark plug can’t ignite the air/fuel mixture properly; it leads to unburned fuel to come out of the exhaust. This results into strong fuel smell coming out from the exhaust pipe.

What Does a Bad Spark Plug Look Like?

Typically the bad spark plugs will have glazed electrode tip along with carbon emission deposits around or on top of the center electrode. You will also notice the gap between the electrodes is either too close or too widened up.

This makes it harder for the spark plug to properly ignite the gas-air mixture in the cylinder which results in poor mileage.

If the spark plug has burned too much due to running too hot, you will notice blisters on the insulator tip. You may also find that electrodes are badly broken off or flattened that is irreparable and only demands new spark plug.

What Bad Spark Plugs Sound Like

The bad spark plugs would result in uneven burning of fuel, with the RPM also following suit. These would cause a loud, grating noise. In other cases, one would feel vibrations starting in a low manner, and ending up shaking the vehicle.

This may occur even though the vehicle is in an idle position or moving very slowly and these are bad spark plug symptoms. Such vibrations can be traced to the engine and it shakes the vehicle.

What Happens If You Drive With Bad Spark Plugs

Driving with a bad spark plug causes loads of problems. Keeping aside the general issues such as decreased engine power, poor mileage, and hard engine starting, the bad spark plug can cause serious damage to the engine especially to the catalytic converter of the car. This can happen when the faulty spark plug fails to ignite the air/fuel mixture and brings the engine to a complete halt.

Is it Safe to Drive with Bad Spark Plugs?

No, it is not safe for anyone to drive a car, when the vehicle is misfiring, as this would damage the engine. You should definitely avoid driving your vehicle with a misfiring cylinder for maintaining its performance for the long run and should change the bad spark plug as soon as possible.

How long Can You Drive With bad Spark Plugs?

It is difficult to say how long you can drive the vehicle with bad spark plugs. However, the bad spark plug still ignites the air/fuel mixture with a weak spark and makes the engine run, but it keeps igniting the mixture until the catalytic converter gets damaged. Once it gets damaged, the vehicle’s motor would not start at all and eventually, the vehicle would not start.

Generally, the spark plug lasts up to 80,000-100,000 miles, but a bad spark plug requires immediate replacement, this is only to prevent further damage to the engine.

Will Engine Get Damaged If You Keep Driving With Bad Spark Plug?

Bad spark plugs are capable of causing engine misfires due to imbalances in air or fuel mixture.

In other words, the combustion process is not smooth. If you keep driving with bad spark plugs, the worst failure that can happen is that it can break off the spark plug parts. These broken parts may hit the internal parts of the engine including piston, cylinder, and cylinder head and cause damage to these engine parts.

How Many Spark Plugs Does a Car Engine Have?

The number of spark plugs you need depends on the size of the engine. Majority of engines will require 1 spark plug for each cylinder on the engine. A four cylinder engine will have four spark plugs; six will have six, and so on.

In the case of a V4 engine, there would be only one spark plug per cylinder. Therefore, four cylinders would equate to 4 spark plugs. The situation may vary if the car possesses a dual spark system and that would house two spark plugs in each cylinder.

In the case of a V6 engine, there would be one spark plug for each engine, thus making it six spark plugs. The models with twin spark engines, 12 spark plugs are required.

Where is the Spark Plug Located in a Car?

They are located at the engine end of the wires, and right beneath the plug covers that attach them all. This is where it is generally found. But in the case of an engine with four cylinders, the spark plugs would be located on the top of the engine. If not so, it should be at the sides. Similarly, in a six 6 cylinder vehicle, the spark plug can be found on the top of the engine or the sides.


Spark plugs may be diminutive in size and not expensive at all. But it is the heart of the engine that would propel the vehicle. Even the slightest bit of damage would spoil the entire vehicle, if not attended timely.

Frequently Asked Questions

#1. What is the Function of the Spark Plug?

The primary function of the spark plug is to ignite the air/fuel mixture by means of an electrical spark. The secondary function is to dissipate heat from the combustion chamber.

#2. When is the Electrode Gap Set?

No matter whether you are installing a new spark plug or inspecting the old one, the gap between the center and the side electrodes needs to be set if it is either too close or too widened.

The gap between the center and side electrodes of a spark plug must be at an exact distance; else the plugs would not fire properly.

#3. What is Engine Knocking?

If your vehicle has incorrect spark plug other than what is suggested, this will lead to engine knocking.

#4. What is the heat capacity of the spark plug?

Typically, the heat range of the spark plug refers to the speed at which the spark plug dissipates the heat from the combustion chamber. It varies from engine to engine.

#5. Can Bad Spark Plugs Cause Transmission Problems?

Yes, Bad spark plugs can lead to transmission problems. Generally, a bad spark plug would lead to a misfire, and this may not be from the engine. The car would jerk around on both sides, and as a result, the transmission would not be able to shift up or down smoothly.