Signs of Bad Spark Plugs

Eagle's Garage may collect a share of sales or other compensation from the links on this page. This comes at no additional cost to you, and all the prices and availability are accurate at the time of publishing.

Explosions are happening in your engine! Well, not exactly, but close enough. The combustion engine has many different parts that must work together for it to run. Here’s a guide on learning the signs of bad spark plugs.

The cylinder head houses the valves that open and close to let air and fuel in and exhaust out. The pistons travel up and down inside the cylinders, providing the power to turn the crankshaft.

In between all of these moving parts is a thin layer of oil that helps to lubricate everything and keep it cool.

Bosch 7547 (WSR6F) Spark Plug, Pack of 2

One of the vital parts of the system is the spark plugs that look like little screws with a gap at the end. When the piston reaches the top of its stroke, the spark plugs fire, igniting the air/fuel mixture. The heat and pressure from the resulting explosion push down on the piston, turning the crankshaft.

Spark plugs are designed to last for tens of thousands of miles, but they do eventually wear out. Over time, they can become fouled or damaged, leading to many problems. Here are some signs that your spark plugs may be going bad.

What happens when spark plugs go bad?

Spark plugs going bad may present themselves in a few different ways. The most common sign is engine misfires. If one or more of your spark plugs is not firing correctly, the air/fuel mixture in that cylinder will not ignite. This can cause the engine to run rough or stall altogether.

Another sign of bad spark plugs is increased emissions from the tailpipe. This is because the incomplete combustion caused by a misfiring spark plug can send unburned fuel straight out of the exhaust.

If your spark plugs are going bad, you might also notice decreased fuel economy. This is because the engine has to work harder to compensate for the misfiring cylinders. The engine may run, but it won’t run well. You may notice a decrease in power, acceleration, and fuel economy. In severe cases, it may even stall.

Also Read: A Complete Guide to Spark Plug Socket Size

What causes spark plugs to go bad?

E3 Spark Plugs E3.54 Premium Automotive Spark Plug w/ DiamondFIRE Technology (Pack of 1)

There are a few different things that can cause spark plugs to go bad prematurely. Fouling is one of the most common problems. This happens when deposits build up on the spark plug, preventing it from firing properly. Deposits can be caused by anything from oil to fuel additives.

Another common problem is simply wearing and tear. Over time, the metal components of the spark plug will slowly erode. This can eventually cause the gap between the electrodes to become too wide, preventing a proper spark from forming.

Finally, spark plugs can be damaged by physical trauma. If the engine backfires or runs too rough, the spark plugs can be knocked out of position. This can cause them to malfunction or even break completely.

Also Read: Best Spark Plug Wires in 2022 (Comparisons and Reviews)

What are the Signs of Bad Spark Plugs? 

The following are a few signs that your spark plugs may be going bad:

1. Reduced gas mileage

Some may not notice this, but one of the first signs that your spark plugs may be going bad is actually reduced gas mileage. This happens because bad spark plugs cause the engine to run less efficiently, resulting in increased fuel consumption.

2. Lack of acceleration

As the plugs aren’t conducting electricity or igniting as well, this will be reflected in a loss of power when you press the accelerator. The car may also feel sluggish and take longer to pick up speed.

3. Hard starts

An easy-to-spot sign that your spark plugs have gone bad is if the car takes longer than usual to start up. This is because it’s harder for the engine to turn over when the plugs aren’t firing properly. Although, if it’s just one cylinder that’s affected, it may not be as noticeable.

4. Engine misfires

A very common and even easier-to-spot sign of bad spark plugs in engine misfires. You’ll know this is happening if the car jerks or shakes when idling or during acceleration. This means the engine is firing erratically and is often caused by one or two bad plugs.

5. Rough idling

When the engine is running, it should be smooth and consistent. But if the spark plugs are going bad, you may notice the car idling roughly. This can be accompanied by a shaking feeling inside the car. When the plugs cannot ignite the fuel properly, the engine will run erratically.

7. Engine Knocking

Due to improper combustion, engine knocking may occur. It is a knocking noise that you will hear coming from the engine. The knocking noise is caused by the air and fuel mixture detonating too early in the engine.

8. Exhaust Smells Like Gas

Last but not least, another sign the spark plugs may be going bad is if the exhaust smells like gas. This is caused by the fuel not burning completely due to the spark plugs not working properly. Not only is this a sign that something is wrong, but it’s also dangerous as it can lead to carbon monoxide poisoning.

Frequently Asked Questions 

How often do spark plugs need to be replaced?

This depends on a few different factors, such as the make and model of your vehicle, the type of spark plugs you’re using, and your driving habits. In general, most spark plugs will need to be replaced every 30,000 to 50,000 miles.

Can I drive with a bad spark plug?

It’s not recommended to drive with a bad spark plug as it can cause many problems. In addition to reduced performance and fuel economy, it can also lead to engine misfires, which can damage the catalytic converter.

Do spark plugs affect acceleration?

Yes, spark plugs affect acceleration as they are responsible for igniting the air/fuel mixture in the cylinders. If the plugs are fouled or damaged, they may be unable to do this properly, resulting in reduced power and acceleration.

Is it expensive to replace spark plugs?

The cost of replacing spark plugs will depend on a few different factors, such as the make and model of your vehicle and the type of plugs you’re using. In general, most people can expect to pay between $100 and $200 for a complete spark plug replacement.

Will bad spark plugs throw a code?

Not all bad spark plugs will throw a code, but some may. If the engine is misfiring, it can often trigger a check engine light. So, if you’re experiencing any of the other symptoms of bad spark plugs and your check engine light is on, it’s a good indication that they need to be replaced.

Related Codes:


If you’re experiencing any of the above symptoms, it’s a good indication that your spark plugs may go bad and need to be replaced, such as engine misfires, hard starts, or reduced performance. The spark plugs are responsible for igniting the air/fuel mixture in the cylinders, so it’s important to keep them in good condition.

Don’t forget. It’s important to replace them before they cause any further damage to the engine. Replacing them is relatively simple and inexpensive, so it’s best to take care of it sooner rather than later.

Leave a Comment

Our Best Content In Your Inbox