Spark Plugs 101

March 20, 2025

7 min read

March 20, 2025

7 min read

All gas engines use spark plugs - opens in new window or tab. to combust a gas-air mixture. They’re simple in their design, as well as cheap and easy to replace.

What’s interesting is learning how to read them. There are a few different ways that spark plugs can go bad. Expert mechanics can tell what’s wrong with an engine just by reading the signs of wear on its spark plugs.

How Spark Plugs Work

A close-up view of a spark plug while sparking, on a dark background. — A spark plug converts electrical energy into thermal energy.

A spark plug creates the spark for igniting the air-fuel mixture in the cylinder. This ignition creates the combustion needed to power the engine. In essence, spark plugs convert the electrical energy from the ignition system into thermal energy to drive the engine.

Here’s a breakdown of spark plug components - opens in new window or tab. to explain how they work:

Terminal: Located at the top of the spark plug, it connects the plug to the ignition source that generates the high voltage current to make a spark.
Center stem shaft: Connects the electrode to the terminal.
Ceramic insulator: Ensures the high voltage current doesn’t escape before it reaches the electrode tip. It also supplies thermal conductivity that keeps the plug operating at ideal temperatures.
Electrode tip: Located at the bottom of the spark plug, this tip extends into the combustion chamber to supply the spark for ignition.
Outer shell (housing): Supports the insulator, grounding electrode , and threads that mount the spark plug in the engine.

A spark plug creates a high-voltage spark across the small gap between its electrodes. The ignition system sends voltage to the spark plug, and it is ionized in the air of the gap, creating a conductive path. Electricity jumps that gap, creating the spark that ignites the compressed air-fuel mixture in the cylinder. The combustion from this process creates the pressure to push the piston, thereby powering the vehicle.

For the spark plug to work right, that small gap has to be clean. If your engine is running rich with too much unburnt fuel, it can foul the spark plugs with dark soot. If your head gasket or piston rings are leaking, it can foul the plugs with wet residue.

A fouled spark plug can only produce a weak spark or no spark at all. Even regular use eventually wears down the electrodes, making it difficult for electricity to jump the gap and make a strong spark.

How Long Spark Plugs Last

A mechanic wearing woven gloves compares an old spark plug with a new spark plug, with the car’s engine in the background. — Depending on the spark plug type, it might need to be replaced after 20,000 or 125,000 miles.

Figuring out when to change spark plugs comes down to what kind of plug you buy:

Cooper spark plugs : As the most affordable spark plugs, these tend to last between 20,000 and 40,000 miles.
Platinum spark plugs : While they cost more, some plugs last up to 100,000 miles, making them well worth the price.
Iridium spark plugs : Due to the added strength, iridium spark plugs can last 25 percent longer than platinum, offering up to 125,000 miles of performance.

If your engine isn’t running properly, your spark plugs may be the first things to wear out. Running rich, running lean, or running with the wrong timing greatly cuts down spark plug life.

Signs of Bad Spark Plugs

A female driver wearing a black and white striped shirt looks concerned. — Fouled spark plugs can cause hesitation while accelerating.

Poor engine performance is among the most common signs you need new spark plugs. As the spark plugs wear or foul, you may notice more hesitation or a rough idle. If the situation gets bad enough, your engine can misfire. If a misfire occurs, the check engine light may illuminate due to several codes, from P0300 to P0312. There could also be difficulty starting the engine because of a weak spark.

Expect decreased fuel efficiency, as well as excessive emissions because of incomplete combustion. Additionally, you could hear knocking or pinging if the spark plugs aren’t igniting the fuel completely.

What does a bad spark plug look like?

There is visible wear in a weak spark plug. Inspecting the plug can determine a lot about its state and what caused it to fail.

If a black sooty buildup is on the end, there could be fuel or oil contamination.
Burnt tips with a white or blistering look suggest that the engine is getting too hot.
Greenish corrosion or rust spots indicate electrical issues or moisture.
Excessive wear on the electrodes suggests that it’s time for a replacement.

Can you drive with bad spark plugs?

No. It’s never a good idea to drive with bad spark plugs. Not only do the bad plugs lead to poor engine performance, increased emissions, and bad fuel economy, but other components can also be damaged in the process.

Most importantly, driving with fouled plugs can cause wear to the catalytic converter - opens in new window or tab. . That’s a very expensive fix. For the cost of a new set of plugs, it’s not worth taking the chance.

Replacing Your Spark Plugs Is Easy

A mechanic wearing orange gloves uses a socket wrench to remove an ignition coil in order to replace a spark plug. — Newer vehicles use an ignition coil on each spark plug.

With most vehicles, changing spark plugs isn’t overly complicated. The spark plugs are typically on the top of the cylinder head centered in the valve cover, or they are on the side of the cylinder head between the exhaust ports. Our complete how-to guide and video - opens in new window or tab. covers how to replace a spark plug with detailed step-by-step instructions.

The most complicated vehicles tend to be V-8s or horizontally mounted V-6s, where reaching those rear plugs can be challenging. Extensions are helpful in these situations.

Should you replace all your spark plugs together?

We recommend that all of the spark plugs be changed at the same time. If one plug is worn, the others are likely in the same condition. By replacing the spark plugs together, you ensure even performance and have better protection from engine misfires.

What does it cost to replace your spark plugs?

The cost of spark plug replacement varies widely based on the type and brand, usually ranging from $2 to $20 each. Standard copper spark plugs are the least expensive option but have a much shorter lifespan than platinum or iridium plugs.

However, it’s best to consider the price per mile that you are spending. For example, if you buy a copper plug for $4 and it lasts 20,000 miles, you’ve spent $0.02 for every 200 miles. On the other hand, if you spend $8 for a platinum plug that lasts 100,000 miles, the cost is slightly less than $0.02 for every 200 miles. You achieve better performance for an equal amount of money.

What else should replace when you change spark plugs?

While replacing your spark plugs, you may also want to swap out the ignition coils - opens in new window or tab. . These usually cost between $15 and $100 each. When combined with changing the spark plugs, you perform a better tune-up for increased engine performance. With newer ignition coil designs, additional replacements may be needed. You may need a coil-on-plug (COP) replacement.

Additionally, you may have to remove a spark plug boot - opens in new window or tab. to get to the plug. This boot can wear out over time, affecting the electrical connection. Replacing these with the spark plugs helps to ensure optimal performance. If you desire even more performance, consider some DIY ignition system upgrades - opens in new window or tab. that don’t take long to perform.

Older vehicles use spark plug wires - opens in new window or tab. , a distributor - opens in new window or tab. , as well as a cap - opens in new window or tab. and rotor - opens in new window or tab. . When replacing the plugs, it’s good to inspect these items and replace any that are worn.

About the author

Brian Jones

Brian worked for several decades as an ASE Certified Master Technician in dealerships before starting his used car lot. Today, he writes for top automotive publications including Motor1. In his spare time, he's dreaming of an escape to the country's best trails in an off-road beast.

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This article is meant to provide general guidance only. Automotive maintenance, repair, upgrade, and installation may depend on vehicle-specifics such as make and model. Always consult your owner's manual, repair guide for specific information for your particular vehicle and consider a licensed auto-care professional's help as well, particularly for advance repairs.