Here’s When to Change Your Spark Plugs

While the automotive industry shifts toward fully electric vehicles at this very moment, internal combustion engines have relied on a strike of electricity all along. Changing spark plugs hasn't become a thing of the past, yet. These inexpensive engine parts are responsible for creating a car’s all-important spark to ignite the gas and fuel mixture. If they're old or incorrectly set, they'll cause malfunctioning, or an inoperable, dead-as-a-doornail car.

Spark plugs do require maintenance and replacement, but there’s no blanket rule as to when they need servicing. It’s up to you, the owner, to regularly check your plugs and to recognize signs of a faulty plug. I'm here to show you how.

A spark plug is a small device that plugs into the engine, more specifically into the cylinder, and provides the spark for combustion via a conduit connecting the battery to the spark plug. A household consumer engine typically has as many spark plugs as there are cylinders, so an inline-four has four plugs, and a V8 has eight plugs. Some cars, however, feature two spark plugs per cylinder for more precise ignition. 

Spark plugs possess a bunch of small parts that use copper, nickel, iridium, and platinum, among others. For protection and insulation, spark plugs also use ceramic.

When spark plugs collect build-up or corrode, they become less and less effective. They can even become inoperable if you let them go long enough, leaving your engine down a cylinder with lower performance and efficiency.

A spark plug creates a small string of high-voltage electricity within a specifically measured gap between a center electrode and a ground electrode. This spark occurs within the cylinder and ignites the air and fuel mixture. This explosion pushes the piston back down the cylinder.

If a car’s spark plugs are fouled, improperly gapped, or dead, it could exhibit one or more poor running symptoms. If you remove them and look at their appearance, certain visual cues could tell you it's time to change 'em out.

This usually happens when you have more than one spark plug that needs replacing. If it flat-out won't start and the spark plugs are the problem, then the car is past due for a new set.

It only takes one bad spark plug to affect the smoothness of your engine's operation.

This is an audible clue that becomes especially evident as you push the accelerator.

If your car isn't getting the type of fuel mileage it usually does, the spark plugs are a good place to check first.

Does your car act sluggish trying to climb hills or get up to interstate speeds? Maybe it's time to check the plugs.

If you pull the spark plugs and see they're blackened and dirty, have a large gap between the electrode and body, are cracked, or oily then it's time to toss 'em.

If you're not sure about the physical appearance of your spark plugs, you can always test them to see if they're still alive and well. Or, heck, they might appear to be in good shape, but you want to be absolutely sure that they're firing. For that, we have this great guide.

With the correct tools, checking and changing spark plugs can be accomplished by even amateur DIYers. For a complete breakdown, check out The Drive’s step-by-step guide, How To Change Spark Plugs.

For a spark plug to deliver precisely the right amount of electricity at the correct time, it needs to be gapped correctly. Spark plug gaps can be checked and maintained with a spark plug gap tool, and we’ve got you covered on that too. For more information, read The Drive’s guide, How To Gap a Spark Plug.

It's best to refer to your owner's manual to determine what the proper spark plug is for your car. Sure, they might all look the same, but they have different sizes, styles, ratings, and so on. Asking fellow owners what they run on Facebook groups and online enthusiast forums can be a great place for information, too, but tread carefully as there's plenty of misinformation out there. In our own DIY wrenching, we've encountered these models that work in a myriad of vehicles and are made by what are generally considered to be reputable brands.

A: For commonplace consumer vehicles, a single spark plug will typically cost about $2-20. If you’re changing the plugs on a used Bugatti Veyron, well, be prepared to pay a little bit more. 

A: If you want to use a hard rule for changing spark plugs, rather than changing based on the condition of the plug, replace them every 30,000 miles. However, know that they might fail sooner than that, or a lot later.

A: Yes, as a general rule, it’s better to replace all plugs at the same time to ensure consistent levels of performance. No half-measures in this garage.

A: Driving with bad spark plugs could lead to poor performance, poor fuel economy, misfires, or your car might not even start. At worst, they could cause extra wear to your engine.

A: If the spark plug gap is set to an incorrect specification, you might get misfires or knock or see carbon build-up or excess fuel. Always check if the gap is correct.