5 Signs of a Bad Spark Plug

5 Signs of a Bad Spark Plug
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You probably already know you need to change your spark plugs, and maybe how to do it. But do you know when to replace them? If you don’t read the signs now, you could be looking at more money spent later. What does a bad spark plug look like, and what does it mean? Here’s how to figure out when spark plugs need changing.

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What does a bad spark plug look like?

A “bad” spark plug doesn’t work properly, and the reason for its failure can often be determined through visual clues. What a bad spark plug looks like will depend on what caused it to fail, and they’ll all fail eventually.

Diagram detailing the components of a spark plug.

A spark plug uses electrical energy from the ignition system to create a spark in the combustion chamber. Whether from a distributor or a coil pack, the ignition energy transfers to the spark plug through the metal terminal on the tip, where it goes to the central electrode.

The exterior ceramic insulator keeps the high voltage from escaping, and the tip of the central electrode creates the spark. The arc of electricity between the central electrode and side electrode kicks off combustion in your engine.

Spark plugs have multiple components, and they all need to work properly. Engine vibrations can damage the insulator over time, cracking the ceramic casing. Excess heat can warp the electrode. Bad gas, too much fuel or air and any other number of conditions can wear out the spark plug prematurely. If any part isn’t working, you will have a failing or bad spark plug.

What should a spark plug look like? If yours resembles any of the following, it’s time to change them.

Broken electrodes

Both electrodes can break, causing visible or invisible damage depending on where it happened. If either electrode is broken, the spark cannot jump the gap, meaning it won’t fire. This causes a misfire in the cylinder that you’ll definitely notice. These spark plugs should be replaced with new ones.

Carbon fouling

Soft sooty black deposits indicate an improper air and fuel mix. Check the engine air filter, ignition components or air sensors. Once that is done, clean the spark plugs with fuel injector cleaner or brake cleaner and reinstall them.

Wet plugs

It’s unexpected to see the electrode tips wet after being inside a running engine. It’s counterintuitive but can happen. If you flood the engine by continually trying to start the engine without it firing up, all that gas soaked the plugs and prevents proper firing.

Oil buildup

Instead of gas, wet oil coats the spark plug electrode. That indicates worn piston rings or valve seals, allowing oil into the chamber. The solution is complicated, possibly resulting in a costly full engine teardown and rebuild.

Burn marks

Burned spark plugs indicate the plug got too hot. This could be caused by an overheating engine or lean running condition, ignition problems or the wrong spark plug heat range. This issue takes some diagnostic skills, but either way, you need to replace the burnt plugs.

Corroded electrodes

Some minor corrosion and oxidizing are normal—the metal is exposed to oxygen and high heat. However, severe corrosion can happen if water gets to the spark plugs. They can even rust in place. Use some penetrating oil this time, and next time you change the plugs, spray WD-40 on the threads to keep water and corrosion out.

Worn electrodes

Worn-out electrodes often indicate the spark plugs were not changed on time. This will look like the electrodes are trying to get away from each other, increasing the gap and wearing out the electrodes over time. Other signs to expect here are loss of power and misfiring.

Exterior damage

Visible damage to the exterior of the spark plug shows a previous detonation in that cylinder. The improperly timed explosion damaged the electrode or insulator, causing bends or cracks. Detonation can cause serious engine damage, so get this checked out immediately.

Signs of a bad spark plug

Short of popping the hood and removing them, you won’t physically know if there’s a bad spark plug. So, how does a bad spark plug look when you are driving? Look out for these symptoms.

Rough idle

If the engine idles rough, it’s a sign the spark plugs might be going bad. This is because the issue causes an interruption in the usual firing order, causing excess vibrations. Note this symptom typically improves some as engine revs climb.

Poor acceleration

Combustion is the reason an engine has power. When the combustion process gets interrupted, so does the power. This makes for faltering, stumbling performance or just a feeling of running on half the cylinders. That last part just might be what is actually happening.

Reduced fuel economy

That poor acceleration does more than annoy you. The decreased performance causes you to step on the accelerator harder to get up to speed. Bad spark plugs can decrease fuel economy by up to 30%.


An engine backfire happens when unburned fuel reaches the hot exhaust components and ignites, causing a loud pop or bang. While the fuel burns, it’s happening at the wrong place and time. Take this seriously because backfires are detrimental to engine performance and health.


Similar to backfires, engine misfires happen when the air and fuel mix doesn’t ignite properly, resulting in engine stumbling, poor performance, vibrations and a check engine light. Spark plugs and other ignition components can cause misfires. Spark plugs are cheap and easy to replace, so if your engine is misfiring, the spark plugs are a good place to start diagnosing.

What to do with a bad spark plug

For cleaning or replacing spark plugs, keep it simple. If the spark plug is dirty, clean it. If the plug is damaged or broken, replace it.

Spark plug replacement is easy enough for a DIY home mechanic, or the task averages a few hundred dollars from a pro mechanic.

Also, remember to have the underlying issue checked out. If the spark plugs were simply old, a simple replacement should do it. However, misfires, oil-soaked or burn damage won’t go away by simply changing plugs.

Frequently Asked Questions

Is it safe to drive with bad spark plugs?

Generally speaking, it is not safe to drive with a bad spark plug. While there is little risk of fire or loss of control with a bad spark plug, the loss of performance makes maneuvering in traffic difficult. There’s also the risk of the bad spark plug causing additional damage, which will only get more costly if you don’t replace it.

How long do spark plugs last?

Spark plug life depends on many factors, but you can typically expect around 80,000 miles out of a set in a modern vehicle. You can help them last longer by checking for the proper spark plug gap before they go into the engine, using the right fuel octane recommended by the vehicle manufacturer and keeping the engine tuned up.

Can I change spark plugs myself?

You can absolutely change a bad spark plug yourself. It’s an easy, beginner-level task many people can tackle. The steps vary by the manufacturer and engine, but you need new spark plugs and one spark plug socket on a ratchet, extension and spark plug gap gauge.

Can you clean a dirty spark plug?

Spark plug manufacturers suggest you can clean dirty spark plugs. Use a dedicated spark plug cleaning tool or use brake cleaner or fuel injector, throttle body or carb cleaner and scrub stuck deposits with a copper wire brush.

Can bad spark plugs still spark?

A spark plug with some carbon buildup or damage can usually fire, though with degraded performance. Damaged plugs, such as those with a cracked insulator, usually can’t fire. Again, just because it sort of works doesn’t mean you should continue putting up with subpar performance. Get new plugs.

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About Andy Jensen

Andy Jensen is a former reporter, now automotive enthusiast writer. He covers industry news, manufacturing, car reviews, race recaps, maintenance how-tos, and upgrades. Andy has contributed content to Jalopnik, Advance Auto Parts, Carvana, and zeroto60times.com. His project car is a modified Scion FR-S, but he’s probably looking at $400 beaters on Marketplace right now.

Disclaimer: The above is solely intended for informational purposes and in no way constitutes legal advice or specific recommendations.