When do spark plugs need to be replaced

Spark plugs deliver an electric spark that starts combustion inside the cylinders of a gasoline-powered engine. Without a spark, a gasoline engine would not start. As a part of regular maintenance, spark plugs need to be replaced at some point. When? That depends on many factors, but first let's see why spark plugs need to be changed.
Worn spark plug Worn-out spark plug.
The spark, created by high-voltage impulses, happens between the tip and L-shaped electrode at the bottom end of the plug. Over time, the tip and the electrode wear out. See the photo. As the gap between them increases, higher voltage is required to produce a spark. This adds strain on the ignition system components, resulting in ignition coil or ignition cable failures. A failed ignition coil or cable will cause an engine to misfire requiring a visit to a repair shop.

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 or runs a little rough, however in other cases, there might be no signs. Besides regular wear, spark plugs might need to be replaced if they become fouled up, see this example. This often happens in cars that are driven for only short distances, high-mileage cars, or when the engine consumes oil between oil changes. One of the common symptoms of fouled-up spark plugs is when the engine misfires and runs rough at idle, 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.

Can the condition of a spark plug be checked? Yes, in fact, spark plugs are typically inspected if a car has driveability concerns. The condition of a spark plug can tell a lot to an experienced mechanic.
How long can spark plugs last? There are two types of spark plugs: conventional and long-life (platinum- or iridium-tipped). Conventional spark plugs need to be replaced every 30,000-50,000 miles.
Spark plugNew spark plug.
Recommended replacement intervals for iridium- or platinum-tipped spark plugs vary between 50,000 and 120,000 miles.

To find out the recommended replacement interval for your car, check your maintenance schedule. For example, the schedule for the 2010 V8 Ford Explorer recommends replacing spark plugs at 90,000 miles, while in the 2008 Toyota 4-Runner, spark plugs need to be changed every 30,000 miles. The recommended spark plug replacement interval for the 2012 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.

Is the spark plug replacement expensive? Conventional spark plugs cost $6-10 per plug. Platinum or iridium spark plugs are more expensive: $15-30 per plug. Cost of labor depends on the engine design. To replace spark plugs in a 4-cylinder engine you might be charged $50-110 for labor. In some V6 cars and SUVs, the access might be more difficult. For example, to change spark plugs in the 2010 V6 Ford Escape may cost from $260 to $320 in labor, because to get access to rear spark plugs the intake manifold must be removed. Do all spark plugs need to be replaced at the same time? Yes, spark plugs are replaced as a set.