When do spark plugs need to be replaced

Spark plugs deliver the electric spark that starts the combustion inside the cylinders of a gasoline-powered engine. Without a spark, a gasoline engine would not start. The spark, created by the high-voltage impulses, jumps between the tip of the center electrode and the L-shaped ground electrode at the bottom end of the plug.
Spark plug The spark plug is installed in the cylinder head.
Modern cars are equipped with long-life spark plugs, but they still need to be replaced.

Over time, carbon deposits are building up on the electrodes reducing the spark plug performance. The tip and the ground electrode gradually wear out. See the photo below. As the gap between them increases, a higher voltage is required to produce a spark. This adds strain on the components of the ignition system. Often it results in failed ignition coils and shorted ignition cables. In either case the engine would misfire requiring a visit to the repair shop. Worn out or contaminated spark plugs can also cause poor fuel economy and various driveability problems. In some cases the spark plug insulator may crack and that will also cause misfiring, see this example.

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.
Worn-out spark plug Worn-out spark plug covered by carbon deposits.
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. When the engine misfires, the Check Engine flashes on and of on the dash. Spark plugs might also need to be replaced if they become fouled up. 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 long do spark plugs last? There are two types of spark plugs: conventional and long-life (platinum- or iridium-tipped).
Iridium-tipped spark plug New Iridium-tipped spark plug.
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. We have seen iridium spark plugs lasting more than 150,000 miles, but at higher mileage older spark plugs are more likely to cause ignition coil failures and other problems.

To find out the recommended replacement interval for your car, check your maintenance schedule. 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.

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 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.