Can bad spark plugs cause a check engine light?

Yes. Worn plugs can cause misfires that trigger a check engine light and flashing code. Ignoring misfires can overheat and damage the catalytic converter, so have them diagnosed promptly.

What are the symptoms of bad spark plugs?

Rough idle, hesitation or stumbling on acceleration, reduced fuel economy, hard starting, and a misfire are common signs. If the engine feels less smooth than it used to, worn plugs are a likely cause.

Should I replace all spark plugs at once?

Yes. Plugs wear at roughly the same rate, so replacing the full set keeps the engine balanced and prevents one weak cylinder from causing misfires. Replacing one at a time is false economy.

Do spark plugs affect fuel economy?

They can. Worn or fouled plugs cause incomplete combustion, which wastes fuel and reduces power. Fresh plugs often restore lost fuel economy and smoother acceleration.

T&H Tire And Auto Service

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Spark Plug Replacement: When and Why It Matters

August 9, 2026 6 min readBy T&H Tire And Auto Service

Worn spark plugs cause rough idle, misfires, and poor fuel economy. Learn how spark plugs work, when to replace them, and why ignoring them gets expensive.

What spark plugs do

Each spark plug ignites the air-fuel mixture in its cylinder at exactly the right moment, thousands of times per minute. A strong, precisely timed spark means smooth power, good fuel economy, and clean emissions. As the electrode wears, the spark weakens and combustion becomes incomplete.

Symptoms of worn spark plugs

Rough or shaky idle, hesitation when you accelerate, a drop in fuel economy, hard starting, and a misfire or check engine light are the most common signs. You may also feel a lack of power under load or notice the engine runs less smoothly than it used to.

Plug types and replacement intervals

Copper plugs are inexpensive but wear quickly, often needing replacement around 30,000 miles. Platinum plugs last longer, and iridium plugs can run 60,000 to 100,000 miles. Always use the plug type and gap specified by the manufacturer — the wrong plug can cause poor performance or engine damage.

Why worn plugs cause bigger problems

A misfiring cylinder sends unburned fuel into the catalytic converter, which overheats and can melt its internal honeycomb. A $10 spark plug left too long can turn into a $1,000+ catalytic converter repair. That is why a flashing check engine light should never be ignored.

What else to check with the plugs

When plugs come out, a technician can read their condition — oil fouling, ash deposits, or a melted electrode each point to a different underlying problem. Ignition coils, wires, and the air-fuel mixture should be checked at the same time. Stop by T&H Tire And Auto Service and we will inspect your plugs and ignition system and recommend the right service interval for your vehicle.

Need this looked at in person?

Our team offers free inspections at 3602 Old Hickory Blvd.

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Questions & Answers

Frequently Asked Questions

Common questions our technicians hear about spark plugs.

Standard copper plugs may need replacement every 30,000 miles, while platinum and iridium plugs can last 60,000 to 100,000 miles. Always follow the interval in your owner manual for your specific plug type.

Still have a question? Contact our team or give us a call and we will walk you through it.