When do spark plugs need to be replaced?

Spark plug Spark plug
Good spark plugs are vital for your engine to start and run smoothly. A gasoline engine has one spark plug in each cylinder. Spark plugs are threaded into the cylinder head, so the tip of the spark plug protrudes into the combustion chamber, see the photo.

How does a spark plug work? As the piston approaches the top position, the ignition coil supplies a high-voltage impulse that creates a spark between the spark plug electrodes. The spark ignites the compressed air-fuel mixture, pushing the piston down.
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.

Over time, spark plugs wear out. The gap between the electrodes widens. Deposits, building up on the spark plug, reduce its performance, see the photo below.

Worn out, contaminated spark plugs require higher voltage to produce a spark. This adds extra load on the ignition coil or ignition wires. Worn spark plugs can also cause poor fuel economy and various driveability problems.

Eventually, a worn spark plug can result in misfiring, or in other words, the cylinder with a bad spark plug stops working. An ignition coil can fail too. Many motorists have experienced this problem. A misfiring engine shakes and lacks power. When the engine misfires, the Check Engine flashes on and off on the dash. Driving with a misfiring engine can cause a catalytic converter to melt.
Old spark plug covered in deposits vs new spark plug Old spark plug covered in deposits vs new spark plug.
A catalytic converter is a part of the exhaust emission control system and is expensive to replace.

Spark plugs must be replaced as a part of regular maintenance. There are two types of spark plugs: conventional and long-life (platinum- or iridium-tipped), see the photo below. 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 50,000 and 120,000 miles.

You can find the exact interval in the maintenance schedule for your car.
Conventional versus Iridium-tipped spark plug Conventional versus Iridium-tipped spark plug.
For example, the maintenance schedule for the 2010 V8 Ford Explorer recommends replacing spark plugs at 90,000 miles. In the 2008 Toyota 4-Runner, spark plugs need to be changed every 30,000 miles. The recommended spark plug replacement interval for the 2019 Toyota Camry is 120,000 miles. We posted several links to websites where you can check the maintenance schedule online at the bottom of this post.

Can iridium or platinum 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. In some cases the spark plug insulator may crack and that will also cause misfiring.
Are there any signs that spark plugs are worn out? In some cases, 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 and cracked ignition wires cause the engine to misfire in a rainy weather or after driving trough a deep puddle. In other cases, there might be no signs until a spark plug or one of the ignition components fails and the engine starts misfiring.

Spark plugs might also need to be replaced if they become fouled up.
Iridium-tipped spark plug New Iridium-tipped spark plug.
See this photo. This often happens in cars that are driven for only short distances, high-mileage cars, or vehicles that consume oil between oil changes. One of the common symptoms of fouled-up spark plugs is when the engine misfires and runs rough at idle or when started cold, but clears out after a spin on the highway. It's also recommended to replace all spark plugs if one of the ignition coils fails and spark plugs haven't been changed in a while.

Can the condition of the spark plugs be checked? Yes, in fact, spark plugs are typically inspected if the car has any driveability concerns. The condition of spark plugs 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 plug).
Platinum or iridium spark plugs are more expensive: $15-40 per plug. The labor charge depends on the engine design. A repair shop may charge $50-$110 labor to replace spark plugs in a regular 4-cylinder engine. In some V6 engines, the access might be difficult. For example, to change spark plugs in the 2010 V6 Ford Escape the intake manifold must be removed, 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.