6 Symptoms of Bad Spark Plugs (& Replacement Cost)

The spark plugs ensure the cylinders are firing properly for complete combustion. Here's how to tell your spark plugs are bad

good and bad spark plug

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

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. Let’s begin with the signs:

The most common symptom of a bad spark plug is engine misfires together with a check engine light on your dashboard. You may also notice rough idle, rough acceleration, and difficulty starting your car in the morning.

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.

6 Bad Spark Plugs Symptoms

1. 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.

2. 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

3. 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.

4. 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.

5. 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.

6. 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

The average spark plug replacement cost is between $40 and $600, depending on how many spark plugs you want to replace, car model, and labor costs. A spark plug costs between $10-$30 each and the labor work is between $20-$500.

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 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.

Written by:

Magnus is the owner and main author of MechanicBase. He has been working as a mechanic for over 10 years, and the majority of them specialized in advanced diagnostics and troubleshooting. He has also been a motorsport (drifting) driver for over 5 years.