Bad Spark Plug? Spot It, Fix It, and Keep Your Car Running

Ever turn the key and hear that sad click or feel the engine stumble? A bad spark plug could be the culprit. It’s one of the easiest parts to check, and fixing it can save you a heap of trouble.

Common Signs Your Spark Plug Is Going Bad

First, listen for a rough idle. If the engine shakes or sputters when you’re stopped, the spark isn’t firing right. Next, watch the dashboard. A persistent “Check Engine” light often points to ignition issues, and spark plugs are a top suspect.

Another clear hint is poor fuel economy. When a plug misfires, the engine burns more fuel to make up for lost power. You might notice a dip in mpg or the car feeling sluggish on the highway.

Finally, look at the exhaust. A black, smoky tail can mean unburned fuel is escaping—another sign the spark isn’t doing its job.

How to Test a Spark Plug Quickly

Grab a socket wrench and remove the suspect plug. Give it a visual inspection: a clean, light‑brown tip means it’s healthy. Dark, carbon‑sooted spots, oil fouling, or melted electrodes scream trouble.

If it looks okay, try a spark test. Plug the spark plug’s electrode into a grounded metal piece and crank the engine. You should see a bright blue spark. A weak or orange spark means the plug is failing.

When in doubt, swap the plug with a known good one from another cylinder. If the problem moves, you’ve nailed the bad plug.

Replacing a Bad Spark Plug – Step by Step

Turn off the engine and let it cool for a few minutes. Disconnect the ignition coil or spark‑plug wire—most modern cars use coil‑on‑plug, so just pull the coil off gently.

Use the correct spark‑plug socket (usually 5/8" or 13/16"), loosen the plug counter‑clockwise, and remove it. Check the gap on the new plug with a feeler gauge; most plugs come pre‑gapped, but it never hurts to verify.Insert the new plug by hand to avoid cross‑threading, then tighten it with the socket—about a quarter turn past finger‑tight is enough. Re‑attach the coil or wire, and you’re set.

Start the engine. If it runs smoother, revs up cleanly, and the check‑engine light is gone, you’ve fixed the issue. If problems persist, there may be deeper ignition or fuel‑system trouble.

Tips to Keep Your Spark Plugs Healthy

Use the fuel grade your car recommends. Low‑quality fuel can leave deposits that foul plugs quickly. Also, replace plugs on the schedule in your owner’s manual—usually every 30,000 to 100,000 miles depending on the type.

Regularly clean the spark‑plug area when you do oil changes. Dirt and grime can sneak into the plug threads and cause premature wear.

Lastly, watch for engine knocks or pinging. Those noises often mean the combustion isn’t happening right, and a bad plug could be to blame.

Having a bad spark plug is frustrating, but it’s also one of the simplest fixes you can do yourself. Spot the signs, test the plug, and replace it—your car will thank you with smoother starts and better performance.