Car Maintenance And Repairs

6 Common Bad Spark Plug Symptoms

Mike Cross
Updated Jul 4, 2021
Symptoms Of Bad Spark Plugs

Spark plugs are one of the more important parts of your vehicle. Without them, there’s a good chance that your vehicle won’t properly function as normal.

A bad spark plug might spell trouble for you if nothing is done about them. We’ll be talking about the importance of spark plugs and what symptoms you need to look out for if you are concerned about the condition of them. Most of the time, we won’t know what’s wrong with them until it’s too late.

The Purpose Of Spark Plugs

Spark Plugs are designed to be an ignition for the air and fuel mixture. A spark plug will allow electrical energy to transfer through the plug itself and jump the gap if the voltage is at a high enough level. The ignition will happen in the combustion chamber of your vehicle.

It’s also designed to remove heat from the chamber itself prior to the ignition process. What a spark plug won’t do is produce heat (since it would be a counter to its real intent). The temperature should be low so it can prevent any pre-ignition from happening. At the same time, it should be at the appropriate temperature where fouling is prevented.

The spark plugs are designed to exchange heat from the combustion chamber and transferring it to the cooling system of the vehicle’s engine. You’ll be able to tell a spark plug’s heat range by the amount of heat it can be able to dissipate.

6 Bad Spark Plug Symptoms

Symptoms Of Bad Spark Plugs
Symptoms Of Bad Spark Plugs

If you think you may have a bad spark plug, it might be easy or hard to spot the symptoms. It will be exceptionally tough if you are not mechanically inclined. But thankfully, we’ll list off some of the common symptoms for bad spark plugs. Here’s what they are:

1. Engine Misfire:

If your engine feels like it’s misfiring, nine times out of ten the spark plug will be the likely culprit. This can even cause the engine to idle roughly rather than smooth. Acceleration will also be rough as well. Plus, your fuel economy will likely suffer as well. If anything, your check engine light will likely pop on when this happens. It would be a surprise if it didn’t come at the moment you started the vehicle. Regardless if it does, if you’re concerned about the potential of any engine misfires, you should take it to the nearest mechanic as soon as possible.

2.Check Engine Light:

Let’s say the check engine light does come on. But this is a common symptom for so many other mechanical issues. However, you may need a scanner since it will recognize these problems better than anything else. When engine misfires happen, your engine control unit will likely save a trouble code. From there, you can use the best OBD2 scanner and it will give you the code while you are scanning and troubleshooting the problem.

3.Rough Idle:

Your O2 sensors may think everything is fine. But in reality, it really isn’t. Unburned fuel might play a role in playing a “mind trick” on your O2 sensors thinking it’s processing a regular mixture of air and fuel. But the fuel aspect of the mix is leaner than it is regular. This will lead to a rough idle and more engine misfirings.

4.Rough Acceleration:

This is another issue that will occur in the event of a leaner air/fuel mixture. Not only will idling will be rough, but so will acceleration. The speed of acceleration will probably be slower than normal.

5.Hard Starting:

We weren’t kidding when we say that spark plugs are important to your vehicle. They are essential in the ignition process. A bad spark plug may cause the vehicle to have a hard time starting or never starting at all. Better to start it in five seconds rather than struggle for five minutes just doing the same thing.

6.Bad Fuel Economy

Another issue you’ll face is a suffering fuel economy. This will likely occur with thanks to misfirings and poor O2 sensors. However, a lot of issues can lead to a bad fuel economy.

What Happens When Nothing Is Done?

The smartest thing you should never do is ignore the problem. If you elect to forego taking it to a mechanic, a bad spark plug makes things worse for your vehicle. The more misfires that occur, the better the chance it will cause engine damage. Any unburned fuel that you may have may also leak to other parts of your vehicle (more notably your catalytic converter). Unburned fuel may stick to your catalytic converter and may cause further damage. Keep in mind that replacing them will be costly, so it’s always a good idea to get your vehicle checked out as soon as possible and consider getting the spark plugs replaced.

How To Replace Spark Plugs

Alternatively, if you are more mechanically inclined you can elect to replace to spark plugs yourself. Keep in mind that some spark plugs do differ than others depending on which vehicle you have and the engine underneath the hood. Some engines can be a bit challenging compared to some others (especially if you have a V-Engine).

Here’s how to replace the spark plugs:

  1. First, remove any parts that may be covering the ignition cable or coils.
  2. Remove the cables and coils and then the spark plug
  3. Check the length of your spark plugs. In fact, you should check to see the length of your current (or bad spark plugs) before you even order new ones. Any spark plugs that are too long can cause piston damage.
  4. Spray lubricant on the spark plug threads and tighten them.
  5. Refit the ignition cables and coils.


Bad spark plugs are not an issue that you want to ignore. Especially when it can lead to serious damage to your engine. If your engine is misfiring or if your check engine light suddenly comes on, it’s always a good idea to give it a further look. Pay close attention to any trouble codes that may be related to the spark plugs. Once you are able to confirm the problem, you can replace them by yourself or by way of a mechanic.

Mike Cross
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