Have you noticed that your engine is not running as it should, and the engine light is sometimes showing up?

It may be caused by a bad or failing spark plug. The spark plug is a very important component of your engine and it can cause a lot of strange symptoms and even damage to your engine.

In this article, you will learn what a spark plug is and which symptoms you should look for when you are troubleshooting your engine for a bad spark plug. You will also get information on the cost and how to replace the spark plug.

6 Symptoms of Bad or Failed Spark Plugs

  1. Engine Misfires
  2. Check Engine Light
  3. Rough Idle
  4. Slow/Rough Acceleration
  5. Hard Starting Condition
  6. Poor Fuel Economy

Because of the importance of the spark plugs in the engine, there could be many different symptoms on a car engine when you have bad spark plugs.

Here is a more detailed list of the 6 most common symptoms of bad spark plugs.

Engine misfires

Bad Spark Plugs

The most common symptom of bad spark plugs is that you will feel misfires from your engine. Misfires can cause the engine to run rough at idle, rough at acceleration, and fuel economy changes. The check engine light will probably also show up after a while. You can feel the misfires as the engine will probably be running rough, and the engine sound may not be as it used to be.

Check engine light

Check Engine Light

When the engine is misfiring, the engine control unit will often recognize the misfire and find out where the misfire is occurring and store it as a trouble code and lighten up the check engine light. That’s why it is an excellent idea to read the trouble codes with an OBD2 scanner when you are troubleshooting your car.

RELATED: 6 Causes of Oil on Spark Plugs

Rough idle

Rough Idle

When a misfire occurs due to bad spark plugs, the unburned fuel may trick the O2 sensors. The O2 sensor will then regulate the fuel mixture and you may get a lean mixture. This can cause rough idle and even more misfires on other cylinders.

As you already know now, bad spark plugs cause the cylinder to misfire and this can be especially notible on low RPMs like at idle for example. If your idle is feeling very rough and you car does maybe sound like a tractor and now like usual, it’s a big chance that you are experiencing failed spark plugs.

Slow/rough acceleration

Slow Acceleration

Due to the O2 sensors mentioned before, you can also get slow and rough acceleration from misfires and lean mixture. You will often feel this at hard accelerations, but can also be felt at slower acceleration. If your spark plug is so bad that the cylinder with the bad spark plug isn’t running at all, you can also feel the difference with one cylinder less.

Hard starting condition

Hard Starting

Because of the importance of a good spark in the starting moment of an engine, bad spark plugs may prevent the engine from starting at all or give you a very long cranking time before it will start. If you can also feel an unburned fuel smell when trying to crank the engine, you should check out your spark plugs.

Poor fuel economy

Bad Fuel Economy

Because of the misfires and the O2 sensor regulations, you can get a poor fuel economy from bad spark plugs. This will most likely not be a dramatic difference, but it can be because a misfire can cause a lot of problems for the fuel mixture.

If your notice that your car is running rough and in the same time having a high or low fuel consumption, you should check out your spark plugs.

What is a spark plug?

The spark plug is the component of your engine that is igniting the cylinder fuel/air mixture. The fuel injectors inject fuel into the combustion chamber, and then the piston is compressing the air-fuel mixture. It’s time for the spark plug to ignite the fuel mixture to push the pistons down in this event. If there is a problem with the spark, the air-fuel mixture won’t get ignited, and it will blow out unburned through your exhaust pipe. That event is called a Misfire.

You may understand the importance of the spark plugs and why it’s a very important part of the engine to keep in good shape now.

Spark Plug Location

Spark Plugs Location E1609826980714

The spark plugs are located at the engine’s cylinder head, often under a cover on the top of the engine and under an ignition coil.

In some cases, they can also be located on the side of the cylinder head, which makes them more difficult to replace.

Spark plugs replacement cost

A spark plug cost between 10-30$ each and the labor work is between 20$-500$. You can expect a total of 30-1000$ for a spark plug replacement, depending on the car model and engine type.

Spark plugs replacement cost differs heavily depending on what car model you have. Spark plugs cost 20$-100$ each depending on the model and spark plug type. Spark plugs replacement labor cost can be 20-100$ dollar on a 4 cylinder car engine, but up to 1000$ on a V8 engine or another engine if they are located badly.

Consider that you have a normal spark plug, and the price is 10$ each, and are running a V6 engine, the price of the spark plugs will be 60$. The labor cost differs a lot in different car models. In some cars, you can replace the spark plugs within 10 minutes, and some do require a couple of hours to do the job. But the most common labor cost of a spark plug replacement is around 50-150$ for most cars.