How Often Should You Change Your Spark Plugs?

Spark plugs are one of those maintenance items that, along with things like brake fluid and thermostats, are easily overlooked but extremely important to your car's safety, longevity, and performance. While they're checked at regular intervals when a car is new, once that car's out of warranty, they're rarely, if ever, thought about again. The thing is, like most parts of a vehicle's engine, spark plugs are wear items that will, eventually, need to be replaced. When, though, and why?

First, let's define what a spark plug is, why it's important, and why it fails. Internal combustion engines need three basic elements to work — fuel, air, and spark. Spark plugs, tiny contraptions made of ceramic and various metal alloys, are the business end of a car's ignition system. They transfer a powerful spark from a car's coil (or coil packs) to the engine's combustion chamber. When they fire, they ignite the fuel-air mixture inside the combustion chamber, which makes power. Spark plugs do this thousands of times per minute, for hours at a time, under some of the harshest conditions imaginable.

When a spark plug fails — and this is "when" not "if," because all spark plugs fail eventually — that failure can lead to issues that range from annoying to catastrophic.

The answer to the question, "When should I change my spark plugs" is, unfortunately, a maddeningly vague, "It depends." Like a lot of automotive maintenance issues, the time to change your spark plugs depends on your car, manufacturer recommendations, when and where you drive, the kind of spark plugs you have, and even the quality of fuel you use. That said, spark plugs typically last a long time. A good rule of thumb when dealing with spark plugs is to replace them every 80,000 to 100,000 miles as long as the car is running right, and there are no other issues. 

If your car is new, and especially if it's still under warranty, your dealership or mechanic will inspect your spark plugs per the manufacturer's recommended maintenance intervals and will change them as needed. If your car is older, if and when you need to change your spark plugs will largely depend on how your car is behaving. If a car has a bad spark plug, it'll usually let you know pretty quickly.

So, what are the signs of a failed spark plug? The most common is that your car is hard to start. If the plug isn't providing spark, there won't be any combustion. Other symptoms include rough idling, hesitating or stalling, reduced fuel economy, and misfires. Misfires, especially, will usually trigger a check engine light, but other symptoms of a bad plug are often undetectable by a vehicle's onboard diagnostic systems. Unfortunately, a lot of problems under the hood cause these same issues — things like vacuum leaks or dirty fuel injectors often make your car behave the same way. If it's a bad spark plug, though, you could be doing a ton of damage to your car by running it.

If a spark plug is misfiring, or not firing at all, your fuel injectors — or your carburetor if you're especially cool — are still pouring gas into the combustion chamber. This unburned gas can then wash the oil off of the cylinder walls, which isn't good because the piston is still moving around in there. This can lead to everything from overheating to actual physical damage to the piston rings and cylinder.

As with most automotive issues, an ounce of prevention is worth a pound of cure. If your car starts acting funny or is making noises you don't recognize, get it to your mechanic post haste.