Step 1

Why Do Spark Plugs Need to Be Changed?

When a spark jumps the gap between two electrodes, it actually burns off (erodes) small amounts of metal from each one. Over time, the gap grows to the point where the spark can no longer make the jump. That’s when you get misfires, lower gas mileage, poor acceleration and, ultimately, the dreaded “Check Engine” light.

When to Check & Change a Spark Plug

Not all spark plugs are rated for 100,000 miles. In fact, some carmakers recommend replacement at 30,000-mile intervals. Always follow the spark plug service intervals shown in your owner’s manual to figure out when to replace spark plugs. But if you can’t remember when you last changed your spark plugs, you can pull them and check the gap and their condition. Once you’ve put in the labor to begin checking spark plugs, however, you might as well change them and establish a new baseline for the future.

Check out our favorite car diagnostic tools for DIYers here.

Signs of a Bad Spark Plug

Unlike manufacturers’ guidelines for oil changes, which are overly cautious, the recommendations for how often to change spark plugs tend to be overly optimistic.

For example, if you’ve already got 80,000 miles on a set of 100,000-mile plugs, they’re 80 percent worn and beginning to take a toll on engine performance and gas mileage. Worse yet, after that many miles there is plenty of spark plug wear, and spark plugs tend to seize in the cylinder head.

Removing a seized plug can be costly, especially if the threads in the cylinder head are damaged. So what are the benefits of changing spark plugs? When you consider the gas mileage falloff and the possibility of seized plugs, early replacement makes sense.

Do it Yourself or Take it to a Pro?

The answer depends on the type of engine in your vehicle. Some V-6 models require removing portions of the intake manifold to replace the spark plugs. If you’re not comfortable with that, take your vehicle to a pro. But if your engine affords easy access to the rear bank, you can probably do the job yourself. Just be sure you gap the spark plugs properly and use a torque wrench.

The tools shown are available at online suppliers and auto parts stores. While you’re there, ask the clerk for the spark plug gap and torque specifications for your vehicle. And buy a small packet of dielectric grease. It’s also important to ask about spark plug replacement cost before making an appointment.