Advice

How Do I Know If My Spark Plugs Need Replacing?

By 03/07/2025 8:00am
spark plugs

When To Replace Spark Plugs Quick Tips

  • Spark plug replacement intervals vary from 25,000-30,000 miles for copper plugs to 60,000-100,000 miles or more for platinum and iridium varieties.
  • Symptoms that indicate you might need new spark plugs include reduced acceleration, engine misfires, poor acceleration, and rough idling.
  • Spark plugs are made from various metals, with more expensive materials like platinum and iridium offering longer lifespans than cheaper options. Follow the automaker’s recommendation when choosing spark plugs.

If your gas-powered vehicle’s acceleration has gone from “zoom, zoom” to “putt, putt,” there’s probably a spark plug problem.

Poor fuel economy is another indication the plugs or spark plug wires need replacing. Although modern spark plugs last considerably longer than those produced 30 years ago, they don’t last forever and need regular replacement. It’s best to check the owner’s manual. Read on to learn about these essential components of internal combustion engines and how often to replace spark plugs.

What Does a Spark Plug Do?

A spark plug is a device that uses a small bolt of electricity to ignite the fuel and air mixture in the car’s engine, creating the combustion that moves the pistons and gets your vehicle moving.

One spark plug is screwed into each engine cylinder. It’s needed to start the engine and to keep it running. The biggest demand for an ignition system is to start the car and make it run.

  • The portion of the plug that sticks out from the engine is connected to the vehicle’s ignition system, which must provide a specific amount of electric current to generate a spark inside each of the engine’s cylinders.
  • The opposite end of the spark plug has two exposed electrodes located inside the cylinder. The electrical current from the ignition system travels to the plug’s center electrode. A high-voltage spark jumps a small gap to reach the second electrode.

That spark ignites the fuel-air mixture inside the engine cylinder. Each time a spark is created, a small explosion occurs inside the cylinder that pushes down against the top of the piston. The type of engine tells you how many spark plugs you need. If your vehicle has four cylinders, it has four pistons; six cylinders, six pistons, etc., each served by an individual plug.

A spark plug is a reliable workhorse. For example, at idle, say 800 rpm, the spark will fire 200 times per minute in a 2.0-liter 4-cylinder engine. As engine speed increases, say to 2,000 rpm, the plug fires 500 times per minute. Diesel engines do not use spark plugs.

MORE: What Happens if You Put Diesel in a Gas Car?

Why You Need to Replace Your Spark Plugs

Most spark plugs have a lengthy life and can be changed on schedule with routine maintenance. It’s beneficial to keep healthy plugs and wires to help ensure they fire on time and keep the engine running smoothly. Faulty or worn-out spark plugs prevent the synchronized combustion that makes smooth engine operation.

When to Replace Spark Plugs

There is no single rule of thumb regarding spark plug replacement intervals because their longevity depends on the spark plug type and quality. Less-expensive copper spark plugs have a relatively short lifespan and must be replaced every 30,000 miles or less, while pricier platinum or iridium spark plugs can last up to 100,000 miles.

MORE: Does My Car Need a New Fuel Filter?

8 Signs the Spark Plugs or Spark Plug Wires Need Replacing

In addition to the visible clues mentioned above, there are other indicators for spark plug replacement. 

1. Scheduled Maintenance

Check the owner’s manual for the replacement interval. Many plugs require replacing at 30,000 miles, while newer models with platinum or iridium plugs can go 100,000 miles or more before needing replacements.

2. Spark Plug Wires

Really old vehicles use a distributor, distributor cap, and spark plug wires. Some later models have an ignition system without a distributor but still have traditional spark plug wires. Newer vehicles use a coil-on-plug ignition system that eliminates electrical problems caused by worn-out spark plug wires. The industry now requires more accurate control of spark and fuel delivery because of the need to boost fuel economy and reduce emissions.

Owners of older vehicles with spark plug wires may need to replace them because they become brittle and tend to crack. At that point, they no longer provide the proper amount of electrical current to ignite the spark plugs and adequately burn the gasoline.

3. Fuel Economy Degradation

Dirty or fouled spark plugs reduce fuel economy because the plugs don’t efficiently burn gasoline in the combustion cycle. Miles-per-gallon can drop by as much as 20% to 30%. A mechanic will replace the plugs and adjust each plug’s gap to factory specifications using a special tool.

4. Slower Acceleration

If it takes longer to accelerate and there’s less power to pass a car, for example, the issue could be worn spark plugs that need replacing. However, a bad fuel filter, dirty or clogged fuel injectors, and problems with the oxygen sensor and ignition system can also slow acceleration.

5. Rough Idling

If the engine makes a pinging, rattling, or knock-like noise or has a strong vibration, the spark plugs or the spark plug wires could be to blame.

6. Engine Misfires

Replace the plugs more frequently if oil is on the plug’s tip when removed from the engine. The presence of oil is due to a cracked valve cover gasket, degradation of the spark plug O-ring, a faulty head gasket, or defective or worn valve guides. A repair is necessary because oil can cause the engine to misfire or prevent it from starting. An engine that continues to misfire could damage the catalytic converter, costing about $1,500 or more to replace.

7. Difficulty Starting

Worn-out spark plugs can be the cause. Have a seasoned mechanic determine if the plugs need replacing. Simply, the engine will not start if the spark plugs can’t produce enough spark to start the combustion process. Some of the other causes for difficulty starting include issues with the ignition system, a battery needs replacing, or worn-out spark plug wires. Replace the car battery if it fails to produce enough voltage to start the engine.

8. Warning Lights

Finally, don’t ignore the engine silhouette symbol or the “Check Engine” light. These warning lights may illuminate if the spark plugs fail, or if the spark plug wires need replacement. However, it depends on the automaker’s purpose for the light. It might be limited to warning the gas cap is loose, the vehicle is emitting emissions above U.S. regulations, the oxygen sensor or the mass airflow sensor may need replacing, or the catalytic converter is not operating correctly.

A reminder: If the check engine light flashes, immediately shut off the engine and call a tow truck. The flashing light may signal a serious problem with the catalytic converter. Ignoring this warning could result in an expensive repair bill. Check the owner’s manual to understand your vehicle’s warning symbols.

How Much Do Spark Plugs Cost?

Spark plugs range in cost from a few dollars to more than $25 each, depending on the brand and which type you buy.

If you prefer not to do the work yourself, you can use Kelley Blue Book’s Service and Repair Guide to check the average cost of spark plug replacement in your area for your vehicle and find a nearby auto shop to perform the service. According to Kelley Blue Book data, the national average cost for spark plug replacement service is about $400, across all vehicle years, makes, and models.

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What Causes Spark Plugs To Go Bad?

Spark plugs perform behind the scenes and aren’t usually on a driver’s mind. However, spark plug tips can overheat because of heat buildup in the combustion chamber or a malfunctioning cooling system. Repeated overheating can cause the plug to fail prematurely.

Other reasons spark plugs go bad include leaking oil or the buildup of carbon or other combustion byproducts in the chamber.

What Does a Bad Spark Plug Look Like?

Look for visible signs of a worn, damaged, or faulty spark plug.

  • Cracked insulator. Chips or cracks in the plug’s white ceramic casing can interrupt the flow of electricity.
  • Worn electrode. Erosion or melting around the J-shaped ground electrode and the dome-shaped central electrode at the bottom of the plug indicates it has gone bad.
  • Carbon buildup. Debris build-up on the tip can be removed using a wire brush, but oil on a spark plug might signal an internal engine oil leak.

How Long Do Spark Plugs Last?

Mileage is the indicator for spark plug lifespan; its age in years is not necessarily a factor. Reduced engine performance is a good indicator that might tell you when to replace spark plugs. As with any type of maintenance concern or engine problem, you should follow the guidance in your vehicle’s owner’s manual about how often to change spark plugs.

Types of Spark Plugs

As you probably know, different gas-powered cars boast different engines — meaning they need different types of spark plugs. Depending on the number of cylinders your engine has will determine how many you need. It’s one-for-one. For example, if your car has a 4-cylinder engine, you need four spark plugs.

To go along with engine fit, most spark plugs are made from different types of metal, some being cheaper than others. However, cheap ones tend to have a shorter lifespan than ones made from more expensive metals. Manufacturers will typically recommend the type of spark plugs your gas-powered vehicle needs. It’s best to check your owner’s manual.

Get to know the types of spark plugs below.

Copper Spark Plugs 

Copper spark plugs, around for decades, are the most common and cheapest on the market. However, they tend to have a short lifespan, so you will need to replace them more often.

Iridium Spark Plugs 

Iridium spark plugs offer a very long lifespan, which is reflected in the price. They are often the most expensive spark plug type on the market today. So, if your manual says you need iridium spark plugs, that’s what you need, because anything less might affect performance.

Platinum Spark Plugs 

Platinum spark plugs last longer and typically run hotter. That means these spark plugs reduce carbon buildup in your engine. Since these are made with hard but high-quality metal, you can drive about 100,000 miles before needing to replace platinum spark plugs. If you drive a newer gas-powered car, some manufacturers will recommend this type.

Double Platinum Spark Plugs 

Double platinum spark plugs got their name not from a double coating, but because they are platinum on both the center electrode and the side electrode. These were made specifically for cars with “wasted spark ignition systems,” meaning two spark plugs are fired at once. This causes increased wear and tear on the spark plugs, which is why this type is necessary. You can use a regular platinum spark plug in a wasted spark system, but it will impact both performance and longevity. This type of spark plug also costs more.

Silver Spark Plug Systems 

Silver spark plugs use silver at the electrode tip. These are relatively uncommon and are typically found in older European performance cars and motorcycles. Nonetheless, they are around but are often less durable than platinum and iridium spark plugs. Your vehicle will most likely not need these but always double-check the manual.

MORE: When Do I Need to Change My Transmission Fluid?

Bottom Line on Spark Plug Replacement

Recognizing when to replace spark plugs is critical for maintaining engine performance and fuel efficiency. Key signs include rough idling, sluggish acceleration, engine misfires, difficulty starting, and a noticeable drop in gas mileage. Spark plug lifespan varies significantly by type — copper plugs may need replacement every 25,000-30,000 miles, while platinum or iridium plugs often last 100,000 miles or more. Ignoring worn plugs reduces engine reliability and risks costly damage. For optimal results, adhere to scheduled maintenance, use the correct plug type for your engine, and get professional diagnostics if warning lights appear. Regular spark plug checks ensure smoother operation, lower emissions, and long-term savings on repairs.

Editor’s Note: This article has been updated since its initial publication. Rick Kranz contributed to the report.