Bad Spark Plug Symptoms And How To Fix It (Full Guide)

If you’ve ever owned or reliably used a car, you’ve likely dealt with car issues. And while these can often vary, one of the most commonly reported forms of car trouble comes from it simply not turning on.

In many instances, this will just mean you’ve got a dead or drained battery. However, for a good number of people, the issue is actually due to a bad spark plug.

Bad spark plug symptoms while seemingly complex, are fairly straightforward and pretty simple to point out. Below are 10 signs you’re dealing with a bad spark plug for your car.

10 Symptoms of a Bad Spark Plug

  1. Increased Fuel Consumption
  2. Harder Time Starting
  3. Loss of Acceleration Power
  4. Your Engine Misfires
  5. Your Engine Backfires
  6. Car “Vibrates” When Idle
  7. Engine Surges
  8. “Check Engine” Light Turns On
  9. Engine Hesitation
  10. Engine Knocking

1.     Increased Fuel Consumption

One of the first ways you’ll know whether you are dealing with an issue with your spark plugs is how aggressively your car consumes fuel. Now, there are many potential reasons for an increase in fuel consumption, however, the first culprit should always be the spark plugs.

Low fuel gauge showing on car’s dashboard.

As your car’s engine fails to turn over from complete combustion, it must work harder and consume more fuel to maintain its speed. This is the first sign of potential “knocking”, which we will go over later.

2.     Harder Time Starting

If you’re dealing with a car that has a difficult time starting, the first thing to do is to check the battery. If the battery is still working and functional, you’ll know that the issue is likely a result of faulty spark plugs. Getting them replaced should immediately fix the issue.

3.     Loss of Acceleration Power

If you are dealing with a loss of accelerative power, this is due to the engine not functioning at its optimal potential. As a spark plug’s wire deals with a cut or break, they aren’t able to fully transmit the electrical flow from the battery to the engine. As a result, your car will steadily produce less and less power even if you’re foot is square on the gas.

In fact, over time, you may not be able to even start the car, as the electrical current will fail to even reach the engine. Fixing the spark plugs should fix this issue almost immediately.

4.     Your Engine Misfires

A super obvious sign you are likely dealing with bad spark plugs is a misfiring engine. This most often is the case as a result of incomplete engine combustion. The easiest way to know if your car is “misfiring” is to note whether it stops and starts while driving.

The more often this happens, the more serious the situation is. Don’t let this issue grow into something very dangerous for you and your car, especially since its fix is so small.

5.     Your Engine Backfires

A backfiring engine is due to an imbalance between your fuel and your air. When there is more air in your engine than fuel, you can likely cause a small explosion to occur from the exhaust.

Faulty spark plugs can result in improper igniting between the air and fuel blend. Not only can this result in brief explosions coming from the exhaust, but it can also result in black smoke as well as a strong gasoline smell coming from your tailpipe. Always check the spark plugs before everything else.

A mechanic holding a new spark plug for replacement.

6.     Car “Vibrates” When Idle

The term “vibrate” while slightly odd, perfectly fits what is happening. If your car begins to slightly vibrate while driving or simply while sitting idly, you know there is a potential problem.

As mentioned before, a spark plug issue may not be the reason for this for every person. However, it is an incredibly common reason and something worth considering the first time you start noticing this issue.

7.     Engine Surges

Another sign you’re dealing with a bad spark plug is if your car sporadically “surges” while driving. “Engine Surging” generally happens due to cracked or broken wires outside of the insulation area. This results in an inadequate amount of electrical power flowing, thus resulting in the engine not picking it up.

Changing your spark plugs is one of the fastest and most immediate methods you can employ to fix this issue and get your engine running properly once again.

8.     “Check Engine” Light Turns On

While there are a lot of reasons a “Check Engine” light may come on, it shouldn’t be discounted as a sign there’s an issue with your spark plugs. In fact, if you’ve recently gotten your engine and battery looked at the “Check Engine” light may be all the proof you need that the spark plugs are an issue.

Another great way to use the “Check Engine” light to determine the issue is by squaring it with a lot of the other symptoms on this list. It may not be a smoking gun all on its own, but when paired with a vibrating car that periodically misfires and loses acceleration, you may be able to determine what the issue is.

9.     Engine Hesitation

One of the biggest telltale signs that you’re dealing with a bad spark plug is a hesitating engine. “Engine Hesitation” generally occurs when you are accelerating while driving. This is due to an inadequate amount of electrical current moving throughout the car between the engine and the battery.

Because spark plugs are used to help flow electrical current between the battery and the engine, provided you have a working car battery, they are the most likely culprit you’ll want to check out and have fixed to solve the problem.

10.     Engine Knocking

Finally and most telling of the different symptoms, if you hear your engine “knocking”, you need to have it looked at right away. “Engine Knocking” can be noted fairly easily as the piston heads are required to use more force to achieve the same level of combustion.

Ultimately, while engine knocking isn’t solely relegated to an issue with the spark plugs, it is the first thing you should consider when having it looked at. It should also be noted that, by the time you begin to hear knocking from your engine, the problem will have gone from simple spark plug replacement to needing to take a look at the whole engine itself; so ideally follow the other nine points so it doesn’t get this far.

Tips for Maintaining Your Spark Plugs

Now that you understand the potential issues that can come from dealing with faulty or broken spark plugs, here are several things you can do as regular preventative maintenance for your car.

Be sure to always be on the lookout for the above symptoms and use them as a note when there is an issue worth taking a look at.

Inspect Your Engine Wires

Find good lighting and take a look at the spark plug wires under the hood. In most instances, this process should be fairly straightforward and shouldn’t require more than a visual inspection. You want to be on the lookout for any damaged, cut, or scorched cables or coils. You also want to look for any signs of corrosion.

Run the Engine

You can also run your engine and listen for any strange sounds or noises. Any “whining” or “straining” noises may be a result of a high volt leakage. You also want to consider letting it sit idle. This ensures that you can more accurately hear these noises while also being able to determine if there is any “shaking.”

Spray Wires with Water

Next, you can get a spray bottle and lightly spray the areas where the spark plugs attach to the boots. Ideally, keep the engine off to prevent any potential electrical arcing from occurring.

After cleaning, you’ll want to remove the spark plug and check the boot for any traces of carbon tracking. If carbon tracking is found, you’ll want to consider replacing the spark plugs as soon as possible.

Use a Spark Plug Tester

A misfire or “spark miss” can result in your engine losing power or causing smoke to escape the engine’s exhaust. To test for this issue, follow this simple checklist:

1.     Disconnect the spark plug from the spark plug lead.

2.     Next, connect your spark plug lead with a spark plug tester.

3.     After, connect the spark plug to the tester. This will create a link from lead to tester to plug.

4.     Run the engine and listen for any inconsistent timing or gaps.

A spark plug tester with other tools.

Confirm Wires are Properly Routed

Fairly simple overall, you should follow your manual to determine that all the wires are properly routed to the right port. If they are improperly routed or not secure, simply ensure they are in their correct place as per the vehicle’s manual.

Clean Spark Plugs If Needed

Finally, you’ll want to consider disconnecting your spark plugs from the spark plug lead and cleaning them as thoroughly as possible. This prevents any dirt or debris from getting into the combustion chamber.

To properly perform this, make sure you are using a wire brush, spray bottle, and a spark plug socket wrench.

How to Replace Your Spark Plugs

The method for replacing your spark plugs is largely determined by your car’s specific model. Most standard engine models are relatively easy to replace. Others (like many V-engines) have spark plugs that are harder to reach.

No matter what type of car engine you’ll deal with, the process is largely the same:

1.     Remove all ignition cables.

2.     Remove any parts surrounding or over the ignition cables

3.     Locate and remove the damaged spark plugs.

4.     Ensure that the new spark plugs are the correct length. Spark plugs that are too long can damage the engine’s pistons.

5.     Install the new spark plugs and make sure they are properly fitted.

6.     Lubricate the car’s threads before tightening with torque.

7.     Return all removed parts back into place, including all ignition coils and cables.

What are the Costs of Replacing Your Spark Plugs?

Cost of Parts

Despite the tremendous cost that can come from not having them replaced, spark plugs themselves are fairly inexpensive. Generally, a single spark plug can cost between $4 and $10, based on its quality and material.

Now when people hear that, they immediately expect to pay only $10 for spark plug replacements. What you’ll also have to consider is that you’ll need one spark plug per the number of cylinders your car has. For example, an 8-cylinder car that requires $10 spark plugs won’t have a $10 spark plug price. It will have an $80 spark plug price.

Cost of Labor

From there, you have to add the additional cost which is labor. This will range and vary based on the mechanic in question as well as how accessible the plugs are with your car. You’ll also have to take into consideration whether you’re dealing with an established dealership or an independent repair shop.

Don’t scoff at these variables, as the labor costs can vary pretty substantially, a $50 job in one establishment can cost as much as $200 at another!

Realities of Replacing Spark Plugs

When looking at the realistic costs for replacing your spark plugs, the type of spark plugs used, how many cylinders your car has, and the type of repair shop you go to will all play a role in how affordable or expensive the price can be. These can go from $70 to $300+, so you will want to research what will work best for your car before going to the dealer.

The good news is that even if you are on the pricier side of things, getting your spark plugs replaced isn’t a regular occurrence. Most spark plugs will last anywhere between 10,000 to 100,000 miles depending on their quality. As a general rule, it’s good to have them checked out every 10,000-15,000 miles, just to be on the safe side.

How Does a Car Act When Spark Plugs are Bad?

The biggest difference between a car that has good or bad spark plugs is largely determined by how smooth the car runs when driving If you are dealing with a car that hesitates when you accelerate, constantly fails to start, has frequent misfires and backfires, physically shake when idle, or has trouble maintaining speed then you are likely dealing with a bad set of spark plugs.

Conversely, if your car is running smoothly and without issue then that means your spark plugs are running correctly and there isn’t an issue.


When it comes down to dealing with issues surrounding your car, spark plugs play a vital and essential role in how they operate. While the costs can vary based on the type of car you have, it is important to check them regularly either by yourself or with a professional.

But we want to hear from you. If you’ve got any questions, be sure to sound them off in the comment section below. Have you dealt with a bad spark plug issue, and if so what did you have to do to get it resolved?

Take care of your car and be on the lookout for any of these signs. If your car’s shaking, slowing, or acting strange, consider popping the hood up and seeing if the plugs are still ok; it’ll save you a world of headaches, trust me.

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About Matthew Webb

Hi, I am Matthew! I am a dedicated car nerd! During the day, I am a journalist, at night I enjoy working on my 2 project cars. I have been a car nerd all my life, and am excited to share my knowledge with you!

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