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Bad Spark Plug Symptoms

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Spark plugs are among the smallest and least complex parts of an engine. They’re also critical pieces of hardware. An engine cannot run without them. If even a single plug becomes clogged with carbon, a vehicle will suffer a noticeable loss in performance.

Today, many vehicles come equipped with platinum- or iridium-tipped spark plugs engineered to last 100,000 miles. However, not all will go the distance. The gap between spark plug electrodes can widen over time, and the plug can become dirty or even damaged.

Common Symptoms of Bad Spark Plugs

Here are some telltale signs your car is plugging away with bad or worn spark plugs:

1. Misfiring

Misfiring is the most prevalent symptom of bad plugs. It can cause:

  • rough idling
  • reluctant acceleration
  • surging acceleration
  • vibration
  • engine knocking and pinging
  • dark exhaust smoke

2. Hard Starting

If your engine cranks for longer than usual before starting up, it could mean trouble with one or more spark plugs. Hard starts usually occur when the plugs are having trouble generating a strong enough spark to fully ignite the gas/air mixture in the cylinder.

Have the car checked out before you wind up with a dead battery in addition to bad plugs.

3. Smell of Unburned Fuel

In addition to hard starting, you may notice the smell of unburned fuel as the engine tries to start. This is another sign of failing spark plugs.

4. Check Engine Light

The check engine light will turn on if the engine is misfiring. However, a check engine light could also indicate any number of emissions-related issues, including clogged fuel filters or injectors, faulty oxygen sensors, or a catalytic converter that needs replacing.

If your check engine light comes on, have it taken care of as soon as possible. Even if the engine is otherwise operating normally, getting it checked early can prevent more serious trouble.

5. Increased Fuel Consumption

Been taking more frequent trips to the gas station? That’s another indication your vehicle’s spark plugs need attention. Bad plugs could trigger as much as a 30% reduction in a car’s fuel economy.

Good Spark Plug vs. Bad Spark Plug
Good Spark Plug vs. Bad Spark Plug / Credit: Getty / chas53

Good Spark Plugs vs. Bad Spark Plugs

If you’re handy and have the proper tools at hand, you can pull a plug and inspect it yourself. You’ll want to pay attention to the curved electrode (the side or ground electrode) at the tip where the plug creates the spark. At most, you should see some minor gray, tan, or brown deposits on the electrode. Those deposits indicate the plug is operating normally.

What a Bad Spark Plug Looks Like

If you see black soot at the tip of a spark plug, this points to carbon fouling. Anything from a clogged engine air filter, idling or driving at low speeds for too long, or an improper gas/air mixture can cause carbon fouling. In this case, you must replace the plug. 

A spark plug tip covered in black, oily deposits means there’s likely an internal oil leak caused by worn pistons or valve guides. Though the plug itself may just need cleaning with a wire brush, you should have a mechanic address the underlying problem.

How Long Should Spark Plugs Last?

Heed the automaker’s recommended service schedule and have the plugs changed at the recommended interval. Thanks to the advent of platinum- and iridium-tipped plugs, most of today’s spark plugs are good for around 100,000 miles

Why It’s Important to Replace Spark Plugs

If your vehicle is running poorly and you suspect at least one of the spark plugs is the culprit, it’s best to take the car to a mechanic to have it thoroughly checked out.

How a Car Engine Works
How a car engine works. Credit: Getty / VectorMine

What Spark Plugs Do

Basically, spark plugs use electricity to ignite the gasoline in an engine’s cylinder.

High voltage between a spark plug’s two electrodes (the center electrode and side or ground electrode) causes a spark. That spark ignites the air/fuel mixture in the cylinder. The burning, expanding gas pushes the piston down, which rotates the engine’s crankshaft and ultimately the vehicle’s wheels. 

The more cylinders an engine has, the more spark plugs it has. Some vehicles, especially diesel vehicles, don’t have spark plugs, but most vehicles do.

What Happens If You Don’t Replace Spark Plugs

Postponing spark plug replacement could lead to diminished performance as the spark plug gap between the center and side electrodes begins to widen because of excess wear. If the side electrode should snap off, it could damage the cylinder, which can be prohibitively expensive to repair.

How Much Does it Cost to Replace a Set of Spark Plugs?

It depends on the cost of parts and labor. Long-life platinum or iridium-tipped plugs cost as much as $25 each but can last for 100,000 miles. Older, copper-ended units sell for less, though you’ll need to replace them after 30,000 miles or so.

RepairPal.com estimates it can cost, on average, from $191 to $250 to replace a set of spark plugs, and that’s not counting taxes, fees, or related repairs, including changing the ignition wires.

Spark Plug Wires

It’s a good idea to swap out the spark plug wires along with the spark plugs if the car is older and/or has high mileage. 

Replace Spark Plugs as a Set

You should always replace a full set of spark plugs at a time. Costs vary according to the number of cylinders, a given vehicle’s mechanical complexity, the local wages, and other factors. As with any repairs, you’ll pay the most if you live in a big city and drive a high-performance or luxury model.

Removing a spark plug with a socket wrench
Removing a spark plug with a socket wrench. Credit: Getty / Yacobchuk

DIY: Can I Replace Spark Plugs Myself?

If you’re handy and have the proper tools, you can change spark plugs and cables in an afternoon. Note that it’s easier to perform this procedure in some cars than in others, especially if you have to remove engine parts to get to where the plugs are located. It’s a good idea to search online ahead of time for repair instructions or a video that details the procedure for your particular model.

Tools Required

Assuming you have a clear shot at where the plugs are located, you’ll need a spark plug socket wrench, some basic hand tools, and the plugs and cables themselves. You can purchase spark plugs online or at a local auto parts store. You’ll also want to get a small packet of anti-seize lubricant to spread on the threads to make the plugs easier to remove the next time you need to replace them.


Only work on the engine after it has cooled off. It’s a good idea to disconnect the battery as a precaution. Clean off surface areas to ensure dirt and debris will not inadvertently fall into an open cylinder. To avoid mixing up the spark-plug wires, remove and replace the plugs one at a time.

How to Change Your Spark Plugs

  • Pull up on the base of the spark plug cable to dislodge it from the top of the plug. Do the same where it’s connected to the distributor cap.
  • Use a spark plug socket wrench to remove the plug.
  • Inspect the new plug to ensure the electrode is not bent or damaged. (For easier installation, look for brands that come “pre-gapped” with the proper amount of space between the two electrodes. Otherwise you’ll have to check the thickness of the gap yourself using a spark plug tool at the recommended gap setting.)
  • Carefully screw in the new plug by hand until it’s seated,­ then tighten it with the socket wrench.
  • Press firmly on the spark plug wire to secure it to the exposed spark plug tip and to its connection at the distributor cap.
  • Repeat the process with the remaining cylinders. 

If the plugs can’t be easily removed or reaching them requires more dismantling than you’re comfortable attempting, don’t hesitate to take the car into a shop to have the procedure performed.

Next Steps

If you have questions about this story, please contact us at Editors@carfax.com