Bad Spark Plug Symptoms

  • Aug 17,2023
image of spark plugs image of spark plugs

Many drivers assume that when their car has trouble starting, there’s a problem with the battery. While this certainly could be the case, it’s not the only possibility. Old or faulty spark plugs could be the culprit! After all, these small spark plugs have a big job to do. Learn why it’s important to keep this part of your car in great working order and when to change spark plugs by watching for these bad spark plug symptoms.

What Are Spark Plugs And What Do They Do?

Spark plugs are an insulated plug that are an important part of a vehicle’s internal combustion engine. They screw into the cylinder head that sits at the top of the engine. Only about the size of a finger, spark plugs take in high voltages of electricity and use it to ignite the air-fuel mixture in the combustion chamber. The combustion process, burning fuel with oxygen, is what powers the engine.

Because spark plugs bring power to your engine, they have sway over your car’s engine’s performance, efficiency, and even emission levels. But over time, like any auto part, spark plugs can wear out. It’s important to spot the signs of bad spark plugs early and replace old plugs to keep your engine working at its best.

Signs Of Bad Spark Plugs

1. Your check engine light comes on

Worn/failed spark plugs, coil packs, or spark plug wires can trigger your check engine light to come on. In today's cars, if a spark plug fails and can no longer ignite the air-fuel mixture, then the most obvious event should be the check engine light coming on or even flashing.

This can cause your engine to misfire and your check engine light to pop on. A flashing engine light could mean that potentially catastrophic misfires are taking place, which could damage the catalytic converter(s).

In general, it's best to replace spark plugs as part of preventative maintenance based on the manufacturer's suggested maintenance schedule. This can help save you from costly repairs, as driving with misfiring spark plugs could put undue stress on your car's catalytic converter (the engine's exhaust cleaner).

Symptoms of misfiring spark plugs may include rough idling, uneven power when accelerating, and an increase in exhaust emissions.

Note that a check engine light could be potentially caused by a number of things, not just a spark plug problem. That's one reason your local Firestone Complete Auto Care offers Code Scans, which can reveal codes that are related to the issue at hand. Since our facilities provide complete auto care, our technicians also have the proper tools and expertise in engine control systems, sensors, and onboard diagnostics to perform an advanced diagnostic and provide an accurate repair estimate.

2. Your car has trouble starting

Your spark plugs have one of the most important jobs in your vehicle, which is to provide the spark that powers the engine! Old, worn out spark plugs have a harder time creating the spark that actually powers your engine. If your car stalls when you are trying to turn it on, there could be a problem with spark plugs or damaged spark plug wires. The battery is most likely the culprit if your car has no power and won't start at all.

3. Bad gas mileage

If your spark plugs are on their last leg, they’ll take it out on your gas mileage. Worn out spark plugs can increase your vehicle’s fuel consumption because they won’t effectively burn the fuel that goes into the engine, meaning you’ll be shelling out more money on fill-ups.

"A misfiring spark plug can reduce fuel efficiency by as much as 30%," reports the National Institute for Automotive Service Excellence.

In general, keeping your car tuned up can work wonders for your fuel efficiency.

"Fixing a car that is noticeably out of tune or has failed an emissions test can improve its gas mileage by an average of 4%," reports

4. The engine idles roughly (and you can hear it!).

Typically (hopefully!) your engine probably purrs like a kitten, but turn down your music and take a listen. If you notice that the engine is making rattling, pinging, or knock-like noises, your spark plugs might be to blame.

5. Your car won’t accelerate quickly.

You’re probably pretty familiar with how your vehicle handles and drives. If you notice that your car isn’t as responsive as normal, particularly when you’re trying to accelerate, this could be traced back to worn spark plugs. It might be time to have them replaced – no one likes driving a sluggish, fuel-guzzling car.

How Long Do Spark Plugs Last?

In any car care matter, it's always best to follow the Manufacturer Suggested Maintenance Schedule. Review your vehicle's suggested maintenance schedule to see when to replace your spark plugs. 

Luckily, spark plugs don’t wear out very quickly. You can typically get 80,000 miles on them before they need replacing. But if you notice any of these symptoms, it’s time to get your spark plugs checked out with an engine tune up.

What Does A Bad Spark Plug Look Like?

So, how can you tell if a spark plug has lost its pep? There are a few visible signs you can spot to single out a bad spark plug:

  • Carbon buildup: Look for debris built up around the tip of the spark plug. Sometimes simply removing and cleaning off a dirty spark plug with an abrasive, like a wire brush or Brillo pad, will make it good as new. But, if you notice black oil buildup around the spark plug, this may be a sign of a bigger problem like an internal engine oil leak
  • A worn electrode: At the bottom of the spark plug, the part screwed into the combustion chamber, is a small metal piece that looks like a dome with a J-shaped piece of metal over it. The J-shaped piece is the ground electrode, and the dome is part of the central electrode that runs the length of the spark plug. These electrodes will wear after years of igniting air and fuel in the combustion chamber. Melting and erosion around these electrodes is a sign a spark plug has gone bad.
  • A cracked insulator: Take a look at the insulator. It’s the white ceramic casing extending from the seal to the terminal of a spark plug. Cracks and chips, even as slight as a hairline crack, can prevent a spark plug from controlling the flow of electricity through the terminal to power your engine. A cracked insulator cannot be fixed, and the spark plug will need to be replaced.

Can You Drive With Faulty Spark Plugs?

Continuing to drive on worn out or damaged spark plugs can ultimately cause engine damage, so don’t put it off. Make an appointment at your local Firestone Complete Auto Care so our expert technicians can see exactly what’s going on!

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