Change Spark Plugs: Signs You Need It and What Happens If You Don't

When you change spark plugs, small but critical components that ignite the fuel-air mixture in your engine. Also known as ignition plugs, they’re the reason your car starts and runs smoothly. Most drivers don’t think about them until the engine starts misfiring or sputtering. But skipping this simple maintenance quietly eats into your fuel economy, power, and even engine life.

Every time your engine fires up, the spark plugs deliver a precise electric pulse to ignite the fuel. Over time, they wear out. The electrodes erode, carbon builds up, and the gap widens. When that happens, the spark weakens or skips entirely. That’s when you start noticing rough idling, slow acceleration, or the check engine light flashing. A bad spark plug doesn’t just make your car feel sluggish—it can cause unburned fuel to flood into the exhaust, damaging the catalytic converter. And that repair? It costs ten times more than replacing the plugs.

It’s not just about age. Driving habits matter too. If you mostly do short trips in the city, your engine never gets hot enough to burn off deposits. That’s when plugs gunk up faster. On the flip side, if you drive long distances on the motorway, they last longer. Most manufacturers say 30,000 to 100,000 miles, but that’s a broad range. The truth? If your car feels off, it’s probably time. You don’t need a diagnostic tool to spot the signs—listen to how it runs. Does it hesitate when you press the gas? Does it struggle to start on cold mornings? Those aren’t random glitches. They’re your engine begging for new spark plugs.

And here’s the thing: you can’t fix a bad spark plug with a tune-up or a fuel additive. No amount of premium gas will clean worn electrodes. You have to replace them. Some people think they can wait until the car breaks down. But waiting doesn’t save money—it just shifts the cost to bigger, avoidable repairs. A misfire can damage the cylinders. Wet fuel can wash oil off the cylinder walls. That’s engine wear you can’t undo.

That’s why the posts below cover everything you need to know. You’ll find real stories from drivers who ignored the signs, what happened next, and how they fixed it. We break down the exact symptoms to watch for, what tools you actually need, and why some shops push unnecessary replacements. You’ll also see how old or cheap plugs affect engine performance and fuel economy—down to the exact miles per gallon you’re losing. And if you’ve been told your car doesn’t need new plugs for another 20,000 miles? We’ll show you why that advice might be wrong for your driving style.

By the end, you won’t just know when to change spark plugs. You’ll know exactly what to look for, what to avoid, and how to make sure your car runs like it should—without overpaying or guessing.