When do spark plugs need to be replaced?

What do spark plugs do in a car? Spark plugs ignite the air-fuel mixture in a gasoline engine; read more on how they work below. Why do spark plugs need to be changed? There are two types of spark plugs: conventional and long-life. In conventional spark plugs, the electrodes are simply wearing out over time. Long-life spark plugs have platinum- or iridium-tipped electrodes that last much longer, but even long-life spark plugs need to be changed at certain mileage.
Old spark plug covered in deposits vs new spark plug Old spark plug covered in deposits and a new spark plug.
One of the reasons is that carbon deposits accumulating on the electrodes of the spark plugs make them less effective at higher mileage, see the photo.

How often do spark plugs need to be replaced? Conventional spark plugs need to be replaced every 30,000-50,000 miles. Recommended replacement intervals for iridium- or platinum-tipped spark plugs vary 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 find the type of spark plugs in your car, check the 'Specifications' section in the owner's manual for your vehicle.

What happens if you don't change worn-out spark plugs? Eventually, worn spark plugs can cause misfiring, or in other words, the cylinder with a bad spark plug will stop working.
Often, worn or contaminated spark plugs cause ignition coils to fail. This happens because as the spark plug electrodes wear out or become covered with deposits, a higher voltage is required to deliver a spark. This overloads ignition coils.
Spark plug Spark plug
Modern cars have a separate ignition coil for each spark plug.

Should spark plugs be changed earlier as a precaution? If your dealer recommends changing long-life spark plugs at 40,000-50,000 miles and they are due at 100K, it's too early, especially if the car runs fine.

Can long-life spark plugs last past the recommended intervals? Yes, we have seen iridium spark plugs last longer, but at higher mileage old spark plugs are more likely to cause ignition coil failures and other problems.

Are there any signs that spark plugs are worn out? You might notice that the engine doesn't start as easily as before. It may run a little rough or stumble on acceleration. In some cars, old spark plugs can cause the engine to misfire, especially in rainy weather or after driving trough a deep puddle. In other cases, there might be no signs until the ignition coil fails and the engine starts misfiring.

How do you know if your car misfires? When an engine misfires it means that it runs on fewer cylinders, for example on 3 out of 4. A misfiring engine shakes and lacks power. The Check Engine light blinking repeatedly on the instrument panel also indicates that the engine misfires. Driving with a misfiring engine can cause more problems, including a melted catalytic converter, which is an expensive part of the emission control system.

Spark plugs might also need to be replaced if they become fouled up like in this photo. This often happens in cars that are driven for mostly short trips, or in high-mileage cars that consume oil between oil changes.
Iridium-tipped spark plug New Iridium-tipped spark plug.
One of the symptoms of fouled-up spark plugs 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, in fact, 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 cheap ($6-10 per spark plug). Platinum or iridium spark plugs cost around $15-40 each. The labor charge depends 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 the $260-$320 range.

Do all spark plugs need to be replaced at the same time? 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? It's recommended to replace all spark plugs if one of the ignition coils fails and spark plugs haven't been changed in a while. 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 the spark plugs? If a car has spark plug wires, it's usually recommended. Old spark plug wires often become brittle and tend to crack. Usually it only adds an extra cost for the wires.

Can spark plugs be cleaned? In old cars, it was a common maintenance job to clean the spark plugs and adjust the gaps between 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 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 at the bottom end of the spark plug.
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 gasoline engine, during the compression stroke, a piston moves up compressing the air-fuel mixture in a cylinder. As the piston approaches the top, the engine computer triggers the pre-energized ignition coil. This sends the high voltage to the spark plug and creates the spark between the center and L-shaped electrodes. The spark ignites the compressed air-fuel mixture, pushing the piston down.