When do spark plugs need to be replaced?

If your car starts well and runs smoothly, it means that you've maintained it well. It also means that the spark plugs in your engine are working. Spark plugs have been a vital part of a gasoline engine since its invention. What do they do? They ignite the air-fuel mixture in the cylinders.
Spark plug Spark plug.
Why do spark plugs need to be replaced? Over time they wear out and may cause problems. Eventually, old spark plugs fail.

What are the signs that spark plugs are worn out? In some cars you might notice that the engine doesn't start as easily as before. It may run a little rough or hesitate on acceleration, especially when cold. Old, worn-out or contaminated spark plugs can cause poor gas mileage. Often, though, there are no symptoms until one of the old spark plugs fails causing the engine to misfire. The term "misfire" means that not all cylinders are firing up. An engine that misfires runs rough, shakes and lacks power. The "Check Engine" warning light flashes on and off or stays on solid.

Often a worn-out spark plug can cause an ignition coil to fail. An ignition coil is the part that generates high-voltage impulses to create an electric spark. See more on how spark plugs and ignition coils work below.
How often should spark plugs be replaced? There are two types of spark plugs: conventional and long-life. Conventional spark plugs need to be replaced every 30,000-50,000 miles.
Iridium-tipped spark plug New Iridium-tipped spark plug.
Long-life iridium- or platinum-tipped spark plugs need to be changed between 60,000 and 120,000 miles. You can find the exact interval in the maintenance schedule for your car. We posted several links to websites where you can check the maintenance schedule online at the bottom of this post. To know what type of spark plugs are in your car, check the 'Specifications' section in the car owner's manual.

Should spark plugs be changed earlier as a precaution? It depends on the mileage. For example, if your car has long-life spark plugs that are due at 100,000 miles and your dealer recommends changing them at 50,000 miles, it's too early. If your engine is good on gas and runs like new, consider waiting until closer to the recommended mileage.

Can long-life spark plugs last past the recommended intervals?
Old spark plug covered in deposits vs new spark plug Old spark plug is covered in deposits.
Yes, we have seen iridium spark plugs last longer, but at higher mileage old spark plugs are more likely to cause ignition coil failures. Long-life spark plugs have more durable electrodes, but they often fail because carbon deposits accumulate on the electrodes, see the photo.

Spark plugs are also often replaced if they become fouled up like in the next photo. This often happens when a car consumes oil between oil changes or when it's driven mostly for short trips, so the engine doesn't have a chance to fully warm up.
Fouled spark plug Fouled spark plug.
One of the symptoms of spark plugs that are fouled up is when the engine sputters and runs rough at idle or when started cold, but clears out after revving or after a spin on the highway.

Can the condition of the spark plugs be checked? Yes, spark plugs are typically inspected if the car has some driveability concerns. The condition of a spark plug can tell a lot to an experienced mechanic.

How much does it cost to replace spark plugs? Conventional spark plugs are not very expensive ($6-10 per spark plug). Platinum or iridium spark plugs cost more ($15-40 each). Labor charges depend on the engine design. A repair shop may charge $60-$110 labor to replace spark plugs in a regular 4-cylinder engine, which is a fairly simple job. In some V6 engines, the access might be difficult. For example, to change spark plugs in a Ford Edge or Escape with a V6 engine, the intake manifold will have to come off, bringing the labor cost to $260-$320.

Do all spark plugs need to be replaced at the same time?
Spark plug set Spark plugs are usually replaced as a set.
Yes, spark plugs are usually replaced as a set. It's important to use correct parts, as wrong spark plugs may cause problems. It's also vital to carefully inspect new spark plugs before installing. If a new spark plug has been dropped on a hard floor, the insulator might be cracked. Cracks in the insulator are visible on close inspection, see this photo.

Do all spark plugs need to be replaced if an ignition coil fails? If one of the ignition coils fails and spark plugs haven't been changed in a while, it's a good idea to change them all as a precaution. If spark plugs are fairly new, replacing one spark plug in the affected cylinder might be enough.
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Do spark plug wires need to be replaced together with spark plugs? Yes, It's usually recommended. Old spark plug wires become brittle and tend to crack. Of course, many modern cars don't have spark plug wires.

Can spark plugs be cleaned? In old cars, it was a common maintenance job to clean spark plugs and adjust gaps between the electrodes. In modern cars, long-life spark plugs are not serviceable and are simply replaced.

How do spark plugs work? There is one spark plug in each cylinder. Spark plugs are threaded into the cylinder head, so that the lower tip of the spark plug protrudes into the combustion chamber, see this photo.
Spark plug The spark jumps between the tip of the center electrode and the L-shaped ground electrode.
An ignition coil can be placed right over the spark plug (coil-on-plug) like in this illustration, or connected to the spark plugs through spark plug wires, like in this photo.

In a four-stroke gasoline engine, during the compression stroke, the piston compresses the air-fuel mixture in a cylinder. As the piston approaches the top, the engine computer triggers the pre-energized ignition coil. The coil generates a high- voltage impulse that creates a spark between the center and L-shaped electrodes of the spark plug. The spark ignites the compressed air-fuel mixture, pushing the piston down.