6 Symptoms of Bad Spark Plugs (And Replacement Cost)

Spark plugs, present in all gasoline-powered internal combustion engines, have one very important purpose: to ignite the air/fuel mixture within the combustion chamber to start a car and keep it running. They are also designed to transfer heat away from the combustion chamber to prevent pre-ignition (or igniting the fuel too early). 

Making sure the spark plugs in your car are in good condition is crucial for it to operate properly. The tiny bolt of lighting each spark plug delivers every time a piston completes a cycle (at top dead center), is necessary to ignite the mixture of compressed air and fuel.

Every cylinder of a vehicle has its own spark plug – a four-cylinder car has four spark plugs, a six-cylinder has six plugs, and so on. When even one spark plug is bad or dirty (fouled), the vehicle’s performance can suffer.

Fortunately, there are some symptoms you can watch for to help diagnose faulty spark plugs. Replacing bad spark plugs is low-cost and often fairly straightforward (though it does depend on where the spark plugs are located in your vehicle).

*Diesel engines are still combustion engines, but they use glow plugs instead of spark plugs.

How Do Spark Plugs Work?

Spark plugs use electricity from the battery to create a spark that ignites the mixture of fuel and air in the combustion chamber. At the end of the spark plug there is a gap which electricity arcs across to form the spark. 

The gap needs to be clean and measured to the correct distance in order for the spark plug to work properly. Over time, the plug may become dirty (which inhibits the flow of electricity) or the ends of the plug can wear so that the gap may be too wide.

Some car owners “gap” their spark plugs, or bend the ends to form the correct distance for electricity to arc. This should only be done by knowledgeable car owners to avoid further damage. 

Eventually, the spark plugs will get so dirty, corroded, or worn that they need to be replaced. Each manufacturer has a specific interval at which spark plug replacement is recommended, so make sure to follow that for optimal performance and life of the engine.

If you neglect the maintenance or if the spark plugs are unusually dirty or broken, the vehicle will start showing signs of bad spark plugs to warn you. 

Types of Spark Plugs

Spark Plug Heat Range

Spark plugs are referred to by their heat range. This describes the ability of the spark plugs to disperse heat away from the combustion chamber. “Hot” spark plugs are those with less heat-dispersal characteristics, and “cold” spark plugs are those that dissipate heat better. 

Hot spark plugs have a longer insulator tip in order to keep the heat closer to the tip of the plug so that carbon deposits that accumulate over time can burn off. If there is too much heat retained, then preignition can occur (which is bad).

Hot spark plugs are a good choice for vehicles that run a rich air/fuel ratio (more fuel in the mixture) because these spark plugs are more likely to have a lot of buildup from the excess fuel. They’re also often the plug of choice for passenger vehicles as the maintenance interval is longer.

Cold spark plugs have a shorter insulator tip which allows more heat to exit the combustion chamber and go into the surrounding water jackets (for cooling the engine). Their tips are more likely to become fouled, or dirty with deposits.

These are often the choice for engines that run lean (less fuel in the fuel/air ratio) or those that run at a high temperature (because that makes preignition that much more likely). 

Electrode Material

Another way to classify spark plugs is by the material that the electrodes (which form the gap) are made of: iridium, copper, nickel, or platinum. 

Copper or nickel is usually the choice for older vehicles or high-performance vehicles. The lifespan tends to be shorter than the others (usually no more than 50,000 miles) but they are usually less expensive. 

Platinum spark plugs are designed for durability, to last longer and run at hotter temperatures (preventing buildup). A platinum disc attaches to one of the copper electrodes to limit wear.

Iridium is a rarer metal sometimes used for spark plug electrodes. These plugs are able to have a smaller diameter center electrode, which decreases the amount of electricity needed to create a spark (more efficient operation). Iridium plugs also still work in extreme conditions (heat, cold, etc). 

Iridium and platinum spark plugs can last anywhere from 60,000 to 120,000 miles, though it depends on the engine, the driving conditions, the fuel, and other factors. Consult the owner’s manual for the recommended replacement interval. 

Double iridium or double platinum spark plugs are designed for coil pack/wasted spark systems (in which each spark plug fires twice per cycle of the engine, once from each electrode). These need the protective metal on each electrode as they both wear faster. 

When choosing a replacement spark plug, it’s important to consult the vehicle manual or professional auto parts store. Choosing the wrong spark plug can cause early wear, inefficient operation, or damage. 

Top 6 Bad Spark Plug Symptoms

There are several indications that your spark plugs are deteriorating that you can look out for as you drive your vehicle regularly.

If the engine throws a code (if the check engine light illuminates), you can connect an OBDII scanner to the port in your vehicle (either at home or for free at most auto parts stores) to see what the code was. This will often indicate which cylinder misfired, if that’s what happened. 

1) Difficulty Starting Engine

trouble starting car

Since a strong spark from the plugs is necessary to start the engine, one of the most obvious indications of faulty spark plugs is that the car struggles to start. If the engine takes longer than it should to turn over, at least one of the spark plugs could be fouled up and need replacing.

Keep in mind that spark plugs do not usually go bad all at the same time. If your car does not start at all, the battery may be the culprit.

2) Engine Misfires

driving problem

Misfires occur when a combustion chamber does not have a good spark or compression, leading to stumbling or otherwise rough engine performance. A bad spark plug can cause incomplete burning of fuel (or even none at all if there is no spark) in the combustion chamber.

If you notice intermittent sputtering or stumbling sounds from the engine, it may be due to a faulty spark plug or some type of sensor malfunction.

In many cases when your engine misfires, the “check engine” light will also illuminate on the dashboard.

3) Bad Fuel Economy

high fuel consumption

Worsening gas mileage could be the result of the spark plugs deteriorating, since they lose the ability to burn fuel efficiently. This is a great reason to proactively check your fuel economy every time you fill up with gas (use the trip meter on the dashboard to do some quick math with the gallons of gas your vehicle just took to fill up, and clear the trip meter each time you get gas after calculating the miles per gallon).

If you suddenly notice a drop in gas mileage, it would be a good idea to check it out, starting with the spark plugs. 

4) Rough Idle

rough idle

Your engine should purr like a kitten (stay at a consistent RPM) when it idles. If you notice the RPM dropping or surging or the car vibrates too much, the car’s computer (ECU) may be trying to compensate for bad spark plugs.

Inspect each plug and replace as necessary.

5) Poor Acceleration

gas pedal hesitation when accelerating

If your car does not respond like it used to when you step on the gas, worn out spark plugs may be to blame.

While sluggish acceleration can be due to a number of reasons, start troubleshooting with the simpler things, in this case the spark plugs.

6) Strong Fuel Smell From Exhaust

burning smell

If bad spark plugs are not properly igniting the air/fuel mixture in the combustion chamber, unburnt gasoline will make its way into the exhaust system. A gasoline smell from the tailpipe will likely be noticeable.

Most drivers have no reason to smell gasoline from the exhaust, so it should be checked out. If you don’t intend to run your vehicle “rich”, then it may indicate a problem.

What Does a Bad Spark Plug Look Like?

If the spark plugs on your vehicle are easy to access, pull them out one at a time by disconnecting the coil pack or pulling on the end of the spark plug connector. Don’t pull on any wires so that you don’t rip them.

Check one plug at a time, so you can put it back in its spot if it looks ok. Use a socket wrench to remove the spark plug from the plug socket. 

A normal spark plug will have a little bit of brown or grayish deposits on the side electrode. 

Black sooty buildup is carbon buildup, and can have a number of potential causes. Replace this damaged spark plug. 

Black oily buildup is caused by an oil leak somewhere. It’s important to find the oil leak and fix it, as many other problems can be caused by oil being where it shouldn’t be (or not being where it should be). Solve the problem and then replace the spark plug. 

If the spark plug looks wet, you may have tried to start the engine too many times without it starting. Wait for the plug to dry before reinstalling it in the car, or clean it with a special spark plug cleaning tool. 

Burned spark plug tips (running too hot) can have “blisters”, white sediment, or appear melted. Address the root cause (with the help of a mechanic if needed) then replace the damaged spark plug. 

Worn, corroded, or broken electrodes are a result of wear and the spark plug should be replaced. Broken ones could even be caused by the wrong spark plug being installed. 

Spark Plug Replacement Cost

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iridium spark plug

Spark plugs themselves are one of the most inexpensive components on a car. A single spark plug will usually cost between $2 to $20 depending on the electrode material. But, since the amount of plugs you need is dependent on how many cylinders your vehicle has, you will need to multiply that figure by 4, 6, 8, or even 10 in some cases.

Standard spark plugs made of nickel alloy are less expensive than premium spark plugs which are made of iridium or platinum.

The average cost of spark plug replacement in vehicles with easily accessible spark plugs is $200.

Some spark plugs are much harder to reach, such as transverse V6 engines and Subaru boxer engines. For these engines, the labor costs can be substantially higher.

The biggest factor in cost is the accessibility of the plugs and whether you’re going to a dealership or independent auto repair shop.

All together (parts and labor), expect to pay anywhere from about $300 to $800 to have your spark plugs professionally replaced. The price will largely depend on the make and model of the car, as some cars require higher quality spark plugs.

Can You Replace Spark Plugs Yourself?

DIY spark plug replacement

Yes, spark plug replacement is often a do-it-yourself job. Most vehicles have easy enough access where even a beginner can change their own plugs. Depending on your experience level and difficulty of access, it may take you anywhere from 20 minutes to a couple hours to do the job.

Before you attempt DIY spark plug replacement, validate that the spark plugs need to be replaced. If the spark plugs look good and you are still experiencing issues, do some more diagnostics. 

The way the electrodes look or if there’s any oil on the spark plugs can indicate other issues that need to be resolved so that the new plugs don’t simply get fouled up again.

Preventative Maintenance

For the long term health of your car’s engine, follow the manufacturer’s recommended maintenance schedule.

While bad spark plugs can cause a variety of alarming problems, they are often fairly easy to diagnose and replace yourself. Use your owner’s manual and careful research to figure out the process for your car, or have a mechanic check it out. Fix the issue so your engine stays as healthy as possible and you save money in fuel and engine maintenance long-term. 


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